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.
But the battles don't work quite well because it's the Rome 2 system. So while it's not as bad as Warhammer or Rome 2, it's not good. The fish scale formation does not work last time I played