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.
The main writer of ME dropped off during the writing of ME2 and they abandoned a lot of his ideas.
I loved the ME trilogy, but after ME1 it really wasn’t about the themes and ideas, just the characters.
Also looking back they’re some of the most lib propaganda games ever like jeez