Virtue. Really, this question is asked so much there needs to be a sticked post saying it...
Mind you, Protector also has a fair RP crowd. Nowhere near as massive a one, but still good all things considered.
Well, Protector's RP crowd is more of the "Author" variety, but they're there. Some of them really do get into character, too.
But Virtue is regarded as the "Constantly In-character" server.
My Stories
Look at that. A full-grown woman pulling off pigtails. Her crazy is off the charts.
This might not be the proper forum for this question, and I apologize in advance, but which servers have the heaviest roleplayer presence for both heros and villains?