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.
3K isn't fantastic but damned if I don't absolutely love it. A good mix of historical themes and over the top gameplay in Romance mode, and the unique diplomacy system only for 3K absolutely rules
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