I had the most enjoyment in Warhammer as well as far as mmo pvp is concerned. I'm sort of amazed they still haven't gone free to play yet.
As far as pvp only mmmorpgs, there are a few that have tried. Forge is out right now that has MMO type combat but is strictly just pvp. I'm not sure how well it will do. There was another game a few years ago that tried something similar called Fury but it failed pretty spectacularly. It only last 10 months before it was shut down. I thought the original idea of Fury was pretty decent but the implementation was so mismanaged. That and the fact that there really isn't a big market for MMO type pvp are factors in why it closed down.