For me it's Diablo II. Granted I've played my fair share of D2 since launch, and also recently on a private server with a comrade from hexbear, but I still feel like years later the game didn't grab me as much as D1 did.
Granted I don't hate D2, but for a game that I keep coming back to, D1 takes the prize.
My issue is when fun pve powers were nerfed because of pvp crap I didn't care for in the first place.
That's why FFXIV has entirely different action bars and rotations for each class in pvp: to avoid that.
I used to love using a huge two handed hammer with Windfury, sometimes even oneshotting people in battlegrounds (yes, I pvp'd but only there because arenas were stunlock meta-hells) until forced skill tree railroading required lava lash offhand mechanics.