Having played all the souls games apart from Bloodborne with a PvP focus I think DaS2 has the best PvP in the series, with my only gripe being that the usual PvP hotspots are pretty dull. I might have sunk more hours into DaS1 PvP, but that's only because I loved being a Forest Hunter and because it was my first souls game.
That said, I haven't played DaS3 since completing my first playtrough. Maybe they have improved on the PvP. My main problem with it was the fact that fights seemed to devolve into R1 spam too easily, and tended to drag on due to estus being pretty hard to punish. Invasions also weren't much fun especially since souls dropped at the place you died at. Also never could get any Watchdogs of Farron invasions, which was a huge disappointed as I was looking forward to reliving those carefree Forest Hunter days.