A great villain is always a beautiful thing. Villainy in its many forms has been a part of video games since at least Donkey Kong, so I'm curious to hear some of people's favorites/who they think are the best. In no particular order, here are some of mine:
GlaDOS-Yeah, yeah, an easy pick, and she's essentially a funnier SHODAN, but think about it. Without her, Portal would be remembered as a nifty little puzzle game to kill a few hours with. "Nah, fuck that," said Valve. "We want to make this something special." And so she taunts you all the way, unintentionally spurring you on to beat her games. To wipe the figurative smile off her figurative smug face. She even starts to break towards the end, Ellen McLain's voice growing more emotional and desperate: "You don't even care, do you?!" It may be aided by electronic futzing, but McLain really sells this character.
But Valve didn't stop there. They took her to a very interesting place character-wise for the sequel (I *love* the barely contained rage and bitterness on her first line: "Oh, it's you."), especially when the tables are turned and you have to begrudgingly work with her to stop Wheatley (another fantastic villain).
Kefka Palazzo-OK, so he's pretty much The Joker if he was even more of a nihilist in a fantasy setting. But he fucking wins. It's still one of gaming's nastiest shocks when he kills Gestahl and splits the world into pieces. Add in that creepy-as-shit laugh, some terrific lines ("You all sound like chapters from a self-help booklet!" is my personal favorite), and you've got an eternal video game villain.
Bowser-Bowser can actually be quite effective as a comedy villain when he's played for laughs, as in the Paper Mario games, and his stuff in Galaxy was surprisingly badass. And his design is classic.
Saren-He's just such a prick. Fred Tatasciore does a fantastic job of giving the character the sense of conviction and belief that he's doing what's right for the galaxy, but the character and performance is never more fun when he's being a lying, space racist douchebag: "Your species needs to learn its place, Shepard."
The Illusive Man-Martin Sheen could've easily phoned this in. With the terrific writing, it would've been fine if he just slummed it. But no, he actually acts his ass off, giving The Illusive Man all the confidence and aged-like-a-fine-wine swagger he requires, but also the calm moral certainty, the anger, and even a certain dry sense of humor. It might be one of my favorite Sheen performances now.