A memorable villain for the good reasons: Saren Arterius.
He's a well-written character with defined goals, motives, character development, and means of action. The way he's portrayal is well done, he's likeable as a villain, sympathetic without being overdone, and, were it not for him being indoctrinated and hating humans with a passion, would have been welcome on my ship.
A memorable villain for the wrong reasons: Kai Leng.
Kai Leng comes off as a whiny child of a character. With no development whatsoever in the game, no clear motive or goals other than being a puppet for the Illusive Man, his characterization is shallow and meaningless. Existing for no other reason than because Bioware wanted an active enemy with a face, he is kept alive again and again due to plot contrivance, not because he is capable, but because the game demanded it. In fact, the game shows Kai Leng as anything but capable, being easily fought against in close quarters by a man who can barely breathe, who had just made his way through the Citadel sounding like he was on his last leg. His need for constant backup during his fights with Shepard and co speak of how poor his skills really are, and only serve to make the game's insistence of his survival all the more contrived.
There's this video log that you can find while in the Illusive Man's base, where Kai Leng is whining, literally whining, at the Illusive Man about how he couldn't kill Shepard. It sums up Kai Lengs character perfectly.
Saren was a fallen hero, his only goal, that of saving his people, used by Sovereign to turn him against the very people he wanted to save.
Kai Leng was a whiny child with an inferiority complex.
Both memorable, but for opposite reasons.
