@DashVader  
When they did that before, they had the full power of the Elements of Harmony behind them.
 
They didn’t use the Elements. No rainbow beam. And Luna thought that she could take him without Celestia or the Elements. And thought that going herself was less risky than sending Twilight.
 
It’s also worth considering that Twilight, being the physical embodiment of magic, might be substantially more powerful than them
 
Never implied anywhere, and Luna specifically thought that Twilight was less suited than herself.
 
She even brought a dragon
 
Twilight tried to get Spike to leave.
 
It also must be considered that when Celestia and Luna beat Sombra before, he managed to just barely escape
 
He did not. They turned him into shadow and stuffed him into a crevice.
 
and he dragged the empire with him in the process
 
That, he did. But given that Celestia has had at least a thousand years to hone her powers and was specifically shown to know his crystal control trick, I wouldn’t count on him being able to do it again.
 
and they also might be concerned about his abilities to block magic, as seen in the heart room.
 
Would they even know about that? From the episode, they hadn’t actually been inside the Empire before, they just showed up and kicked his rump once.
 
Kicked it so hard that he turned intangible, I might add.

@McGack  
Well, another thing to consider is the possibility that Celestia and Luna can’t beat him now. When they did that before, they had the full power of the Elements of Harmony behind them. Now, they don’t. It’s also worth considering that Twilight, being the physical embodiment of magic, might be substantially more powerful than them even before ascending, meaning that she would be more qualified for any magical feats required(such as a localized reversal of gravity- again, that can’t possibly be easy), in addition to, along with the friends she brought along, having the Elements. She even brought a dragon confirmed to be able to eat straight through most crystals like they were made of processed cheese and melt steel, which could be useful in escaping potential traps made of crystals(which there were). It also must be considered that when Celestia and Luna beat Sombra before, he managed to just barely escape, and he dragged the empire with him in the process(and, if it’s more than just that city, apparently destroyed or left some of it as well). They probably don’t want to risk that again, and they also might be concerned about his abilities to block magic, as seen in the heart room. They might well think that only Twilight, as the physical embodiment of magic, is powerful enough to get past it. If so, it didn’t work, but they didn’t know that when they planned it.

@Darkmoon  
well yes but cadence was the one who defeated Sombra her magic returned the heart after spike founded it and Cadence got tossed to save him and she took the heart.

@Background Pony #7185  
Enh. Luna liked her odds against Sombra even without Celestia.
 
While Sombra was smart and a good planner, even with his crystal control spell he didn’t seem to be insanely powerful.

@RainbowDashie41  
Actually it took the whole of the Crystal Empire to defeat Sombra.

villain i pick is Lord Tirek  
here’s history  
Nightmare Moon defeated by Mane Six  
Discord defeated by Mane Six  
Chrysalis Defeated by Princess Cadence and Shining Armor  
King Sombra Defeated by Princess Cadence  
Lord Tirek Defeated by the Mane Six  
most is Mane Six then surprise it’s Princess Cadence Luna none Celestia only once Banished Luna to the moon

I think Sombra is more powerful than Chrysalis and NMM. His dark magic is amazing.
 
But I bet people just voted for their favourites eh…

@DashVader  
honestly i thought that Celestia and luna couldve come in at any time and said, welp looks like twilight didnt pass boom see yah sombra, if sombra had to dodge out of the way of Shining armour’s lasers, then he mustve been a threat

@WM-R  
Yeah, that occurred to me after the fact. If there’s a way to edit comments here, I haven’t found it. Basically, my point was that Sombra is one of perhaps two fully competent villains in the show, the other being Tirek, and even he was defeated before he could do much, albeit by magical mystery box of deus ex machina.

@Mad Black  
no food? we saw a unicorn farmer in the same episode

@Sjogre
 
To be fair, Tirek never actually had a chance to do anything like that.

@WM-R  
Discord strikes me as someone who is fairly content as a prankster, doing things for his own amusement.
 
The thing about that is, Discord has been pretty open about the fact that his favorite kind of fun involves screaming. Psychologically breaking the Mane Six? That was the high point of his day.
 
I’d call Discord outright more cruel than Tirek, since he doesn’t actually torment people like that.

Why do you lazy bastards never lnk the polls?

@Background Pony #5E43  
I don’t think Sombra is equal to Nightmare Moon.

In terms of original power, Discord’s by far the most powerful. In terms of the potential power, Tirek blows away the show.  
Also what @Background Pony #5E43 said.

Sombra is definitely stronger and smarter than Chrysalis and equal to Nightmare Moon.

@Sjogre
 
Like I said, Discord is undoubtedly the most powerful villain, but I’d still list Tirek as most dangerous due to the fact he actually seems malevolent in comparison.
 
While easily capable of great acts of cruelty (e.g. discording the Mane Six and outright brainwashing Fluttershy), Discord strikes me as someone who is fairly content as a prankster, doing things for his own amusement.
 
Tirek, on the other hand, strikes me as someone who is simply cruel and desires domination over others. Sure, he didn’t of thinking of using Discord’s powers the same way, but then, if you gave me Discord’s powers, it wouldn’t occur me to use them the way Discord does either. I rate him as more dangerous because he’d be able to drain your own powers from you, and eventually master them.
 
@DashVader
 
I tried to read your post, really I did, but my eyes began to hurt. I don’t mind reading long texts, but breaking it up with paragraphs would’ve really helped.

@DashVader  
TL; but I read it anyway.

@DashVader  

I still wanna see what Tirek’s natural power was.

@DashVader  
Paragraphs are your friend.
 
 
Honestly, the votes seem about right. Discord is extremely powerful. However, he’s not the brightest villain. His chaotic nature usually sees him come up with great plans, but never has a back up plan for the possibility that he might be defeated.

@DashVader  

@DashVader
 
…you might wanna take Composition 101 again, buddy…

Sombra was actually one of, if not THE most dangerous villain. For one thing, he managed to enslave and torture the Crystal Empire to the point where it was painful for them to so much as hear his name. Nightmare Moon, Chrysalis, and Tirek were all beaten before they could do any damage aside from the millenium+(it never actually says what time of year Nightmare Moon was banished, just that she returned on the summer solstice) of psychological trauma Nightmare Moon inflicted on Celestia(she almost never shows any emotion at all, and when she does, it’s usually very mild amusement. The only times she ever shows any other emotion is solemness and occasionally fury when dealing with powerful enemies). The only time she showed any outward sign of sadness was crying while banishing Nightmare Moon. Considering that Luna was the only other being she would have had around for the duration of her life, that’s got to hurt. As for Discord, they beat him so long ago that no one is even aware that he EXISTS until he returns(for that matter, the same applies to Nightmare Moon, although that may have been more on the part of Celestia intentionally hiding it and disguising it as ancient mythology to prevent a thousand years of riots and/or, as the author widely regarded as one of, if not the best in the fandom pointed out, cults supporting Nightmare Moon. As all too many fans have neglected to consider as they proclaim Nightmare Moon as the hero and Celestia the villain, plants need sunlight to live, and they and everything else need the sun’s heat to keep them alive. If you cast them into permanent darkness, the plants starve, if they and everything else don’t freeze first. Nightmare Moon, while her intentions, misguided though they may have been, were relatively benign, would have accidentally committed omnicide. Those of you familiar with Latin roots may recognize this as meaning “murdering everything”. That’s not even paraphrasing, that’s a literal translation. If the planet in the show is round, the sunny side fares no better. As with real tidally locked planets, while the night side freezes to unspeakably cold temperatures, the day side turns to a scorching desert, except rather than being 120 degrees there, it’s 742. With constant sun, the day side turns into Venus as the heat-absorbing water and various other materials boil into the atmosphere, leaving only a narrow strip at the terminator line of the planet habitable, and even then in perpetual semi-darkness unsuitable for most plants. Holy crap that was a long parenthesis section. It might as well be its own comment. Anyway, back to my real comment), as he only exists to that point as a window and a garden statue. However, I’m rambling. My main point is, Sombra actually took over a city-state(or possibly more, they don’t say how big it was before it came back, and the term “Empire” implies considerable size), enslaved the native population despite their villain-vaporizing spinning rock, and then actually took steps to hide it, to the point where anyone going in alone, or in a group that actually stuck together properly, would be unable to retrieve it- AND it alerted him when it was being stolen. It took being a proto-god(anti-gravity spells can’t be easy, and none of the crystal ponies seem capable of magic anyway), bringing an assistant who was told to stay behind(most, upon reaching the Crystal Heart, would have gone for it as a group rather than having one go in and one stay outside the central area, being desperate enough to JUMP OUT A MASSIVE EVIL EIFFEL TOWER with it, and have the alicorn(it also depended on having one of those, of which only 3 existed at the time, and only two when he devised this plan, and the fact that they shows him proves that he clearly expected them to have too much going on in Equestria to deal with him, and knowing him, he probably caused it specifically for that reason, and they probably just figured out he was behind it) exhausted from protecting the city having a husband who though quickly enough to throw her like the world’s biggest and pinkest paper airplane at the falling heart and its carrier AND her still being alert and rational enough to glide for them AND bring them in for a controlled delivery to the pedestal despite being incapable of powered flight at the time, and finally everyone being confident enough in the thing working to activate a device powered by love and hope AS THEIR CITY LITERALLY CRUMBLES AROUND THEM. Also, he was alerted and went for the falling heart, and almost got it. Also, in the first place, he only lost the first time because they called for TWO GODS AT THE HEIGHT OF THEIR POWER to save them, and even that failed. When you think about it, there’s no way any sane and ration person(which by this point we’ve pretty well established that Sombra is) would plan for this. The odds of it actually happening are the sort of thing that powers spaceships in universes where the ultimate answer to life, the universe, and everything is a random number that doesn’t even correctly answer the ultimate question. Also, if not for this insanely unlikely scenario, it’s stated that he could have taken Equestria too, and possibly even THE ENTIRE WORLD- and even if he didn’t, with Equestria fallen, he’s not the sort of villain dumb enough to leave the aforemention gods in any position they can possibly escape from, particularly now that it’s been established that you can steal magic, even god magic, and use it to send said gods to hell. Really, all things considered, Sombra’s defeat, both times, should not have happened. They were ridiculously lucky, and should get a large and well-trained and armed military up and running IMMEDIATELY in case it turns out he’s not really dead.

@Mad Black  
No weather control, no food and no day or night for anypony. So basically equestria would have been ruined in a matter of time by it self.  
Discord was also able to do that… By making the world too hostile, rather than depowering the ponies.
 
And Tirek was in Cloudsdale, Canterlot, Apalousa and Ponyvill in a rather short period of time  
Thanks to Discord playing chauffeur. Discord was also capable of affecting multiple locations at the same time, something that Tirek never tried.