I'd always been a fan of Hitmontop due to its design before I'd ever used them. Now I've used them all, and I have to say I don't have a favourite anymore.
Although their designs aren't my favourites, I don't dislike Hitmonchan and Hitmonlee. Plus, Hitmonchan is named after my favourite Asian :)
Hitmonlee's kicking attacks have the better animations, Hitmonchan has the best movepool for in-game with those elemantal punches, and Hitmontop is a bit of a tank (despite low hp) and has a decent movepool too.