Well, it depends on what series
Classic: Protoman, Tornadoman, and Nitroman, not necessarily in that order.
X: Definitely not X, And aside from Zero, i kinda like Axl, when he doesn't sound like a girl.
Zero: That would have to be Harpuia.
ZX: This is a tie between Aile and Ashe. The main thing that keeps me from liking Vent mainly involves his temper tantrums. Ashe, however, gets bonus points for smack-talking Vent.
Battle Network: Ooh, so many navis. First off is Elecman, by far my favorite appearance-wise. But the grand winner has to be Tomahawkman because of appearance, the cheery personality, and let's face it: he's just too damn useful in battle.