The name that springs to mind is "Svidrigailov," from Crime & Punishment. Probably just because that's my favorite book, and tends to top all lists I make. But I like him because his character is analyzed—even in books that are pretty good about depth of character, the "villains" rarely get their fair share, when they tend to be the ones whose motives you most want to know about! But in C&P he actually gets some first-person perspective all to himself; we get to see inside his head. So he's a good—I mean, bad—one.