The upcoming release of Age of Conan has brought this question farther into the light, with its foray into real-time battle. Sure, there have been others -- the late, not-so-lamented Fury being a great example -- that have tried to redefine what MMO combat can be like, but many games are content to offer the tried-and-true click to attack model, with slight variations.
When it comes to fighting in games, surely the biggest defining factor is, or should be, "Is it exciting?" Given that singular criterion, then, which game has the best combat? Which one gets your blood racing? Which MMO really lets you, as the kids used to say, throw down?