Champions Online type customization.
Seriously light-years ahead of other things. Sure the game itself was not particularly good, but the character creation engine was almost good enough to justify that. Being a game without gear, your appearance was entirely open to what you wanted it to be. Every single part of your body had hundreds of options for physical design and coloring. You could literally be just about anything you wanted - and if the game had actually been good and survived, they just would have kept adding MORE designs over time.
On top of that, it had my favorite open-ended skill system ever created. I should preface this by saying it was horribly imbalanced and did not end up working well, but the concept itself was solid. There were classes with skill sets, but you did not have a choose a class. You could pick any skill from any class and make your own totally unique build.
Telekinetic auto attacks, summon lightning bolts, have a Shoulder Cannon, use super strength to throw rocks, all on one character. It was awesome on a level I have never seen since.