I like bosses that don't actually turn out to be villains at all, and while their actions might be less-than-heroic, they are more interested in their own agenda that doesn't involve you but are fine with teaming up with the heroes and stopping whatever larger-scale threat that pops up. This is especially so when they stick with you in the long run and become an actual friend (Whether they want to admit it or not!).
There are also those ones who just want to fight you without any malicious intent, facing you in battle is their idea of fun times! A Prankster who can get in your way, but is generally harmless. They can also be sizing you up for later encounters, seeing what you're capable of so they can assist you accordingly later.