Not just in every MMO, but pretty much every RPG. The Paladin (or appropriate variant) is my default character. It's been my favored class going back to old 1st Edition D&D. It's a character concept I know well, so it's a good basis to see how the class plays in a game.

This is not to say I won't play other classes. Started with the Paladin in WoW, but my Warlock became my favorite character. But starting with one "default" class, I can see how a game compares with others that I've played. Sure, not every game has a Paladin class. But most do have something similar. It's a classic.