The games that I keep on going back to are Banjo Kazooie and Banjo-tooie on the Nintendo 64. They're not obnoxiously easy, but it's not hard to get 100% (except for that stupid jiggy in Banjo-tooie. Seriously, I hate Canary Mary).
Just returned to playing FFX again. Idk what it is, but it's such an addicting game to me. Maybe it's just the characters or the skill trees and stuff like that. @_@
Viva Piñata, of all things. I sold my copy of few years ago, only to buy another one a few months later. There's something about it that keeps me coming back. Maybe it's the collecting and completing aspect that appeals to me so much.
Banjo Tooie is a game I always end up going back to for one reason or another. Whether to challenge myself to a boss rush of those games, the neato minigames/multiplayer games, or just playing through the story again.
I'm surprised to see so many others say Banjo-Tooie is a game they continually return to, as it's also mine! It was one of my all-time favorites when I was younger. I also love to replay the majority of Zelda games, mainly A Link to the Past, Skyward Sword, and Majora's Mask.
RuneScape. Just kidding, that game is dead to me thanks to Jagex. Fire Emblem Warriors. I'll play it obsessively for a bit then get bored with it. It is grindy and repetitive, but it's still got a certain charm to it.
