I like my childhood game most i:e Mario. Mario is the game which i played lot,even i end the game then i played start again from the 1st level.
The Super mario world in the Super Nes would always be a classic game. It really made me solve a lot while playing this game.
Monkey Islands. Is it classic? I can't play modern 3D games as they give me a bad headache, maybe I'm just too old! Haaha!
How about Heroes of Might and Magic series, classic, old, yet they eat up time like no other game I've ever played. I rememeber spending several months with HoMM 3 along with the two expansion packs. Gotto love it.