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.
Medieval 2 was the best Total War. I have so many fond memories of trying to break the game in different ways. Making heresy the majority religion, doing opposite colonialism by only sending diplomats to the New World to gift all of Europe to the Aztecs, getting the pope to declare a crusade against himself. It was great.
A new build of eb ii for medieval 2 dropped like a month ago. Haven't played it but it says it solves many of the stability issues thar were due to lack of ram utilization.