I always thought of hybrid as class that does fights with physical weapons and can cast magic spells. I guess in WoW, that is just about every class, but the term did not originate in WoW. I my mind, I would never put physical classes such as rogues, hunters, or warriors in there, or any wizards. DKs would be a hybrid.
I think the best example of a hybrid class would be paladin, which was, as far as I know, a hybridization of the cleric and fighter classes from D&D. You had a limited number of spells from the cleric spell book, but used the fighter's base attack bonus tables. Ranger (or hunter in WoW), was a hybrid of fighter and druid classes. The term hybrid came from the fact that the class was literally a hybrid of two classes.
Using the more-than-one-role definition used by other in above posts, that would make priest a hybrid. I just don't see that. But if that's what we're going with, then priest.
haha, priest best hybrid class trololol.