I really like the passive PvP mechanics in Final Fantasy XI. The actual PvP is rather lacking and leaves a lot to be desired, but the whole territory control element taking place as a passive PvP element in the majority of the world is in and of itself pretty interesting. It’s the sort of thing I would love to see more games adopt; it’s not that control is vital or utterly rewrites the zones, but it does make a difference as different nations control different spreads of territory.
For that matter, I suspect some people who don’t like the usual environment of PvP would enjoy the game’s more passive approach to the concept, even if the mechanics of the game as a whole don’t match what you’d like to play. So which MMO gets closest to your ideal PvP mechanics? It may not be your game of choice, but which MMO has a PvP implementation that feels like the most fun to you?