Yeah, kind of a sappy topic, but I'm sure everyone here has just ONE game that is although not the best game they own (not by a longshot) but for one reason or another always makes you smile and just... well, feel good.
For me its hands down Donkey Kong Country 2. Its probably because at this point I can probably play some of the levels blindfolded, but something about the graphics, audio (oh god the audio), and gameplay just make me... happy? I dunno, this is a fairly esoteric topic.