Since this is an image about Sombra, I’m going to copy-paste something that led me to completely change my outlook on him as a character / villain. It should prove interesting.
 

 
This is the tragedy of Sombra as a character: He had the potential to be the most terrifying thing the M6 ever faced, because he doesn’t fuck around. NMM did that whole ‘my adoring subjects’ thing, followed by a weird testing-their-bonds thing; Discord just messes with them for giggles; Chrysalis lets prisoners group up and basks in her victory before it’s secure.
 
Sombra, though? First thing he does on showing up is put Shining Armor permanently out of commission, then proceeds to wear down an alicorn who ought to be at the peak of her power by sheer persistence. He out-magics Twilight, singlehandedly lays siege to an entire city, and came incredibly close to winning his objective.
 
And it was all done in by bad writing and design.

I don't entirely agree. NMM in my opinion or Chysalis are much scarier then Sombra. They are actually able of contesting and defeating an alicorn. Sombra never really stood a chance. Luna and Celestia could have defeated him again if needed. 

 
If we acknowledge every villain falls victim to poor writing, NMM was the scariest. She is a sadistic type of evil. She knows you can’t beat her, so she toys with her prey. But most of all, she was pissed. Her ponies didn’t respect her, or even know who she was. She sister betrayed her. I don’t think NMM would have killed Twilight and the M6 if she one. A SiM type of punishment is frighteningly plausible.

Head canon wise and after the fact analysis wise, sure. 

 
I just meant as far as episode feel goes. NMM, Chrysalis, and Discord were cartoon evil laugher type villains. You may be able to objectively show that they were more of a threat, but Sombra had this powerful “the evil outside” feel to him. It’s slowly coming for you, and when it gets there, it’s going to be really bad… and no one seems to be able to stop it.

I... I never even realized how menacing he actually was... probably because of how ridiculous he looked when he finally showed up. 

 
Now that i think about it, Sombra could very well be the most successful villain so far. The first time NMM showed up, she got wrecked in about five minutes, the second time it was about two days. Chrysalis might have been infiltrated in Canterlot for weeks, but her invasion failed in less than a day. I’m not sure how long Discord ruled the first time, before he was defeated by Luna an Celly, but the second time it just few days again. Tirek got all OP, but his career lasted for less than a week. The’re all super evil and sometimes even successful, but no one of them can manage to hold their victory.
 
Sombra on the other hand, managed to take over an entire empire and then warp it out of reality for a thousand years. And he might have ruled it for all that time. He can disable people’s magic, he can mind fuck ponies (causing them to forget, creating a door of worst fears,…), and his ethereal form is more badass than NMM’s. And he does all that without being an Alicorn. Sombra’s a boss.
 
If only he didn’t have the looks and the track record of an donut-steel OC.