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.
You're not wrong about 4. It's a fantastic game in my opinion but it's so far removed from resident evil, even the games that came after when it comes to the story, it could have been it's own thing and sold a lot less. It was needed at the time for sure, the series was getting real stale real fast but it's a major departure in style and story. I've actually still got my launch GameCube version of the game. I usually play the wii version since I've got both and my GameCube is a wii. I love an arcade shooter and it's kinda the best arcade shooter ever.