I'm a sucker for sort-of-but-not-really villians, aka "they're antagonists, and they might break the law/morality/whatever's considered villianous in this anime, but they're not really EVIL". Like Sora from .hack//SIGN(more of an annoying noob-type(except actually appealing to the viewer
) than an evil person, especially since he's so young), Kaitou Kid from Detective Conan(even without all that Magic Kaito tells you about him, it's fairly obvious that he's not evil, especially when virtually every other antagonist is a murderer, bomber, kidnapper, blackmailer, or something else that's far worse than his antics), and the main Team Rocket trio from Pokemon(if you honestly think they're evil, you missed every single one of their character development moments. Especially those with backstory).
If we're going for villians that are actually evil...well, it really is the personality more than anything when it comes to what characters I like, in general. Having actual reasoning for their evil desires is nice too, even if the reasons are purely selfish/egotistical/whatever. Although anime tends to be good with that anyway.