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.
I'm glad I never got into competitive PvP or hardcore raiding since I had lots of fun as an enh shammy. Sure, the only time I was actually a force to be reckoned with was wielding a Corpsemaker in Level 20-29 Warsong Gulch, but goddamn those Windfury proc multicrits were satisfying. Also Spirit Wolves are cute. When I did some casual raiding I would occasionally get grumbles about my DPS, but whatever, that was never why I played the game. It was just a way to have fun with my buddies...once they quit, there was nothing keeping me there. Glad I got out before they jumped the shark.
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.