Let's see, let's see...So far for Single Player I'd have to start with...
Legend of Zelda: A Link to the Past (SNES) This is also my favorite Zelda game of the series. When I got my Super Nintendo back, this was one of the first games I got for it. As a kid, I remember beating all three profiles, maxing out the HP etc. There's no annoying water levels and while a couple of the bosses are a bit annoying (temple three, light world) none of them are really over-powered or, cheap.
Pokemon (GB, GBC, GBA, DS) There's no wrong answer really. Any of the traditional Pokemon games are like crack to me. Recently, I grabbed Pokemon Leaf-Green for a 12 hour train trip. It really did help pass the time and, I'm still far enough from the end to have at least 12 more hours of play ahead of me. Even if I beat that one on the way, I've still got Platinum, Red and, Soul Silver (JP) to get back into and, through...for the umpteenth time.
The_Blue_Rider said:
Tetris, 30+ years on and still being played
I'm only at about 20...same for Dr. Mario though so both are honorable mentions for me.
As for multiplayer...
I haven't really got any for multiplayer. I haven't played too much recently and it's getting harder to find games that offer split-screen. Actually, you know what game does still have split-screen?
Left 4 Dead (Xbox 360) It was the first game I played with my current GF so there's some bias there. There may be only a handful of levels but the AI Director has really done its job, keeping each match somewhat different than the last and always exciting. Also love playing as the infected. I don't own the second and while I've played it, I just don't care enough for the settings or, characters to rank it higher than the first.
Teenage Mutant Ninja Turtles IV (SNES) It's a Konami beat-em-up from the 90's. It's on the SNES and there's one with a different name on the Sega Genesis, arguably the two most awesome systems ever to have graced living rooms and bedrooms alike. I've never had a bad time playing this game and it's one of the only games that I think was enhanced by the addition of two-player mode. Honestly though, I was only able to beat this one on hard
because of the two-player mode.