Sauron. Main villain of Lord of the Rings, and inspiration for King Sombra. Out of the three big Dark Lords - Palpatine, Sauron, and Voldemort - he's the most evil, and quite possibly the most powerful. But of the three he has the least screentime, and very few lines.
Now, King Sombra has been getting a lot of hate. I know many liked him and are actually disappointed to see him die (though I was fully satisfied with his death he's my second favorite villain after Chrysalis, meaning no offense to Discord of course), but a lot of people called him a lame villain because he doesn't have a lot of screentime, dialog, or personality.
First, I'll say that hate for Sombra isn't the complaint that annoys me the most; actually, I don't really mind people complaining about Alicorn Twilight or Equestria Girls, it's the taking it out on Hasbro (who were only doing that they thought we wanted them to do) and saying it's the end of the show and/or the fandom that annoys me. But maybe Sombra doesn't deserve the hate he's been getting. This is an entirely different matter than with the other unicorn, who's getting hate she doesn't deserve from a different side of the board (and maybe even less Season 3 screentime than Sombra); Sombra is pure evil, but he's the darkest villain in the show, is quite intelligent with his traps, and doesn't screw around, so he deserves more credit as a villain. As one guy said, he's not a bad villain, he's a different kind of villain.
But anyway, now to the Sauron comparison, which more than anything is to say what Sombra has over him. Now, it's no secret that Meghan McCarthy based him off Sauron, as a "dark presence". My good friend Screwball confirmed this with her on Everfree Radio. They both lost their physical form many years ago, they're both extremely powerful, they both try to take over a kingdom, they both have little screentime and dialog, they even have similar background themes and the Crystal Empire under Sombra's control looks like Mordor. Not to mention Sombra's horn looks like Sauron's spikes in the parody flash video One Ring to Rule Them All.
What does Sombra have over Sauron? You're probably all thinking "nothing, right?" Sauron has more screentime and lines than Sombra. Sauron has more lines than Sombra. Well, remember Sombra was villain of two 20-minute episodes, while Sauron was villain of three movies each over two hours. For length of movies, which ratio of screentime or dialog is bigger? (I'm not sure but I might say Sombra)
Now. What does Sauron do in Lord of the Rings? Both Sauron and Sombra were shown in flashbacks; Sombra enslaving the Crystal Empire, and Sauron forging the One Ring, killing Men and Elves including Elendil, and being seemingly killed by Isildur. That's the most Sauron does onscreen in the series (though he might play a more prominent role in The Hobbit, you never know). For the remainder of the Lord of the Rings movies, he's just sitting atop his tower as a flaming eye, and all he does is send his minions to take over the lands and search for the ring. He does have a few confrontations with Frodo and Pippin, but that's about it. He is unable to even participate in the final battle (although it is possible he executed Shagrat for causing the fight in Cirith Ungol, and did keep Frodo and Sam from advancing until he was distracted), and what does he do when he learns that Frodo is in Mount Doom with the One Ring? He turns toward Mount Doom, powerless to do anything, and the Nazgul have to fly over there to try and take it. Moments later, Gollum falls into the lava with the ring, and Sauron is no more.
Now, Sombra. Many complain that he didn't do anything, because for most of the episode he was outside Cadance's barrier. Well, he couldn't at the time, but when the Mane 6 are approaching the empire, Sombra appears and goes after them, probably intending to kill them. He gets a fight scene with Shining Armor and encrusts his horn with black crystals that Cadance's barrier couldn't even destroy, preventing him from using magic. When Cadance's barrier briefly disappears Sombra tries to get in, and when his horn is cut off he spreads dark crystals through the land. And when the barrier finally disappears for good and Sombra can re-enter the empire, he doesn't really have a chance to do much before he notices Spike, and tries to kill him and take the Crystal Heart. And he would have, too, if it wasn't for Cadance (and her wings!). Before he really has the chance to do anything else, Cadance and the Crystal Ponies destroy him.
So, basically, Sombra has a more up-close and in-person (in-pony?) confrontation with the Mane 6 than Sauron ever had with Frodo or Aragorn, neither truly got anywhere near the Eye except through crystal balls or ring power. Sombra regained his physical form before his defeat and did something to try to stop the protagonists, which is more than can be said for Sauron. They COULD have had Sombra stay in shadow form through the whole thing.
Now, another thing: People say he's pathetic because he got defeated by a baby dragon. Well, Sauron was defeated by a hobbit and an extremely pathetic creature, and those two were the ones who directly killed him. Without even trying! Spike retrieved the Crystal Heart, and once he did it took the power of the Crystal Heart, an alicorn, and an entire empire to destroy Sombra. Plus, he later directly kills a King Timberwolf, who survived everything Applejack sent at him while three regular Timberwolves.
And I'm not saying Sauron is a lame villain, I'm just saying Sombra isn't.