Favorite Imperial ship is the TIE Interceptor, and has been for over a decade. I like the look, kind of like a jagged dagger from the side, and high firepower/maneuverability is my default battle tactic for most anything.
Which makes my favorite Rebel ship (and favorite overall) an odd choice: The Y-Wing. It's not terribly fast, but it is tanky, and I still love the look.