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.
Dungeons Keeper.
That mobile game thing.... Ugh
That was such a horrid early example of how phone games were self-sabotaged into grifty monetization as standard practice.
War for the Overworld is a modern Dungeon Keeper if you're looking to scratch that itch.
Thanks!