NES- Dragon Warrior IV
The epitome of the old time RPGs on the system. It took all the fun gameplay elements, added a good story, then divided things in to chapters in the most wonderful way possible. (A very close 2nd is SMB3, but I didn’t play sick to stay at home and play it.)
Gameboy- Pokemon Red/Blue
A really fun RPG. Collecting teammates makes for a surprisingly good system.
SNES- Chrono Trigger
Perhaps the best designed game ever. Amazing story, intriguing characters, and so much fun stuff to do. Time travel kicks ass. (Earthbound is my personal favorite, but its just not as perfect. FFVI and Super Mario World are also close contenders.)
N64- Super Mario 64
It set the standard for 3d platforming. I mean, come on! (Goldeneye, Mario Kart, and Harvest Moon 64 are good, but just not the gaming milestone that SM64 was.)
PSX- Final Fantasy Tactics
A very addictive job/skill system and a deep plot that feels real put this ahead of the rest. (Honorable mentions include Metal Gear Solid, FFVII, and Suikoden 2. Outstanding games, but I calls 'em like I sees 'em.)
GBA- Fire Emblem
A good game, but nothing to write home about. The GBA as a whole is kinda like that. (I’ve played Mother 3 and, while its good, just doesn’t stand up to the second game.)
GameCube- Super Smash Brothers Melee
It improves on the original in every way and was the first game that I had to take breaks from to keep my hands from falling off.
PS2- Shin Megami Tensei: Nocturne
A ton of great games, especially RPGs, but this one takes the cake. One of the hardest games around, but expertly designed. It has Pokemon’s “Gotta’ catch 'em all!” aspect with a mature, post-apocalyptic setting. You’ll fall in love within 15 minutes. (Romance of the Three Kingdoms X deserves mention as perhaps the best strategy game around. Metal Gear Solid 3 also amazes, and of course the Katamari games deserve mention as well. Persona 3 gets some kudos too, but it can’t quite equal its MegaTen cousin.)
DS- Pokemon Diamond/Pearl
The best treatment to date, it refines the small mistakes of the past and introduces Nintendo WiFi, which makes trading and collecting a breeze. (I’d like to mention Izuna: Legend of the Unemployed Ninja as well. Really fun dungeon crawler that you should pick up if you like the genre.)
Wii- Super Mario Galaxy
Platformer in the spirit of Mario 64, but the Wiimote and spherical worlds make it unique.
PS3- Metal Gear Solid 4
Not as good as MGS3, but still the best game on the system. I will admit, though, that I haven’t played many of the games, so grain of salt.
PC- Civilization
Any of the series, but I prefer Civ4. But as anyone who has played can attest, these games are more addictive than crack soaked in heroin-flavored nicotine.