Wow... Where do I begin?
NES:
Contra (this was the first game I played, and it took me over ten years to finally beat it without cheating or dying!)
Super Mario Bros. 1, 2, and 3 (although, the American 2 was actually based off of some other game; Doki Doki Panic Attack, I think... The Japanese 2 was REALLY hard; released on SNES later under Super Mario All-Stars as Super Mario - The Lost Levels, if I'm not mistaken...)
Freedom Force (yeah, Zapper goodness! I used to play this with my mom a lot...

)
The Adventures of Bayou Billy (I remember this a little bit from my childhood; Game Genie, anyone? Good soundtrack too!)
Life Force (good music, good game! Another good one from Konami!)
Teenage Mutant Ninja Turtles 1 and 2; I haven't played much of 3. (Oh man, I grew up on those! So MANY NES games I grew up on! :huh: *nostalgia*)
Kung Fu (man, that game's cool! For some reason, I always liked the third level boss, and his deep laugh if he defeated you! Rumor has it, that you can fight Thomas' girlfriend IF you can beat the game 50 times... I don't personally know...)
SNES:
Chrono Trigger (one of the best RPGs EVER, IMO. But, the translation left something to be desired! There IS a patch that faithfully retranslates it from the Japanese, but you need a ROM too...)
Street Fighter II, and actually, all variants of Street Fighter (Shoryuken!!)
Mortal Kombat 1, 2, and 3 (Finish Him! *bloody mess, horrifying scream* Fatality! Actually, the SNES version of 1 didn't have any blood, but the Arcade did, and the Genesis had a secret code for it - A B A C A B B...)
Pac-Man, and all variants (I actually HAVE an authentic Pac-Man arcade machine sitting at the apartment... Definitely a keeper!)
PS1:
Chrono Trigger (again... Good FMVs, but the thing loads horribly!)
Chrono Cross (oh man, that's a good game!)
PC:
Doom, and all its variants (keep on rockin,' baby! Yeah! Doom's my favorite one, hands DOWN! Still making the occasional map for it, too!)