I should probably put Morrowind on the list...but that'd be like an addict putting "heroine" down as a favorite past time. XD These are rated by "enjoyability" not "favorite". In my mind, "good game" and "enjoyment" are a bit different.
1) Meteos. It was my first DS game, and my first real puzzle game. I could play it for hours when I first got it, and I'm not above firing it up now.
2) Battlefield Bad Company 1/2. Odd choices, but for sheer "I cannot stop laughing I love this", I don't think I've ever come across a better series. Literally, I could play those games and come away laughing every time.
3) Mirrors Edge. Very few things, in my mind, will ever be as fun as the free running mechanics in that game. I never got tired of it, and even beat it on hard.