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.
Tales of Symphonia: Dawn of the New World had an absolutely infuriating story which was too bad since it had one of the most fun pet collection mechanics in any rpg game I've ever played.
Also obligatory Bioshock 2 good, Bioshock Infinite bad
I love that game purely because it's an excellent monster collector and I love monster collectors. I haven't played the original.