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.
Dead Rising just got worse and worse with each outing. I eventually came to like DR2 (thanks to Off The Record) but I couldn't bring myself to care about 3 and when they changed Frank for 4, I gave up on believing the series could be good again.
Love DR1 but I wasn't wowed by what I saw in 2 so I never continued on with the games.