Donkey Kong! hehe
Seriously, probably Civilization II or III, Fallout II and Sim City 4.
I'll also do what "higherFaith" did with a list on the systems and games I have:
Atari 2600: Asteroids (fun, fun, fun)
TI-99/4a: Pac-Man (a pretty decent version all things considered)
Sega SG-1000: Zaxxon (cool isometric Sega classic)
MSX: Vampire Killer (Castlevania)
Nintendo (NES): Super Mario Bros. 3 (true classic)
Sega Master System: Wonderboy: The Dragon's Trap (awesome! You need to play this.)
NEC Turbografx-16/PC Engine: Bomberman '94 (so fun with multiplayer)
Sega Genesis: Shining Force II (very addicting strategy RPG)
NEC Turbografx-CD/PC Engine CD-ROM2: Ys: Books I&II (beats Zelda in my opinion)
Gameboy: Tetris (easy one)
NEC Super Grafx: Ghouls 'n Ghosts (awesome arcade conversion for 1990)
SNK Neo Geo AES: Windjammers (like Pong with a frisbee. SO MUCH FUN!)
Sega Game Gear: Sonic the Hedgehog (fun Sonic game! I prefer the Genesis ones, still)
Super Nintendo: Final Fantasy III (so many great games)
Sega CD: Snatcher (love the atmosphere. There are a lot of great games on the Sega CD)
DOS 6.22: Civilization (so addicting...)
PC-FX: (nothing really that I own. Not a fun system. There are a couple good expensive games....)
3DO: The Need for Speed
SNK Neo Geo CD: Twinkle Star Sprites (same games as the AES, basically)
Sega Saturn: Castlevania: Symphony of the Night (so many great games! I don't have a fav, reallymaybe my favourite game overall)
Sony Playstation: Castlevania: Symphony of the Night (so awesome that I listed it twice!)
Windows games: Sim City 4
Nintendo 64: F-Zero X (love F-Zero games)
Neo Geo Pocket: Sonic the Hedgehog Pocket
Sega Dreamcast: Shenmue II (I love these games! Where is the third?)
Sony Playstation 2: Gradius V (maybe the best horizontal shooter ever)
Bandai Wonderswan: Judgement Silversword: (spiritual sequel to Radiant Silvergun/Ikaruga)
Gameboy Advance: Astroboy: The Omega Factor (one of the best platformers ever)
X-Box: Star Wars: Knights of the Old Republic (WOW!!!)
Nintendo DS: Castlevania: Order of Ecclesia (almost up there with SotN)
Sony PSP: Gradius Collection (yay, Gradius Gaiden on the PSP!)
Nintendo Wii: Mario Kart Wii (it's so fun...)
Sony Playstation 3: The Elder Scrolls IV: Oblivion (also Fallout 3. I just love Bethesda RPGs)