Darth Revan is from the Knights of the Old Republic games, and he is definitely very cool.

MAJOR, MAJOR, Crucial, Key, Game Spoiler:
(SPOILER)
But after being mind-wiped he returned to the Jedi (well, you get to decide in the game whether he does or not, but officially he became "good" again) to stop his former co-conspirator and apprentice Darth Malak from taking over the galaxy.


Hopefully we will get a final KoTOR game to answer the question of whether Revan's turn to the Dark Side was visionary genius on his part or a totally misguided quest for forbidden power.

Actually the KoTOR games have some pretty nifty Darths, but in terms of power I'll go with Darth Sidious from the movies. This guy succeeded in taking over the galaxy and making the Chosen One his puppet. He effectively eliminated the Jedi Order that had survived for thousands of years. How much of that was his Force abilities and how much was simply his intelligence and cunning? It's hard to say, but the results are devastating and unmistakable. His one flaw was allowing himself to grow overconfident over his hold on Vader. Oh, and IMNSHO he pwned Yoda.