I'm a huge fan of retro games, mostly because I grew up playing them. XD I was like 5 when I started regularly playing Doom, Duke Nukem 3D and Wolfenstein. I know, I'm a weird kid xD
Besides those three, my favourite retro games that I played as a kid (and still play from time to time) are:
Commander Keen
Sam & Max Hit the Road
Freddi Fish, Putt Putt, Pajama Sam, Spy Fox and other Humongous Entertainment stuff (What? Don't look at me like that! It's my childhood!)
Monkey Island series
Most of my NES games collection
Prince of Persia
... and a few more, can't remember the names.
Also, I doubt this counts as a retro game, but it's worth a mention:
Neighbours From Hell
Man, I was quite a video game lover back in First Grade xD I just remembered how puzzled I was when I heard that other kids didn't play that much.. I'm not sure whether to be proud or ashamed xD
ANYWAY, what about you guys?