I second diablo 2.
2 other good pvp games are guild wars and ragnarock if you go for guild wars get factions it gets you into the pvp really fast or you can just make a pvp char on the other ones.
1. D2 sucks at pvping since you have to challenge them. Not really consider a MMO
2. I heard guild wars is really good at pvping but not seeing players outside of town sucks. I mean you have to party with them to do quests.