It all depends on the system. I'll name a few of the ones I remember best.
Atari 2600- Pac-Man, Frogger, Pitfall, Galaga, Q*Bert, Berzerk, Dig Dug, Donkey Kong, Joust, and Swordquest.
Colecovision- Ladybug, Donkey Kong, and one other that I can't remember the name of.
NES- SMB, Zelda, Blaster Master, Mega Man, TMNT, Castlevania, Bubble Bobble, Contra, Astyanax, Excitebike, Wizards & Warriors, and Punch Out.
SNES- Zelda, SMB, Blackthorne, Street Fighter 2, Earthworm Jim 2, Final Fantasy(multiple), Mario Kart, F-Zero, Star Fox, Mario Is Missing, and Mortal Kombat.
N64- Zelda OoT/MM, Mario, Hexen, South Park, 1080 Snowboarding, Conker's Bad Fur Day, Mario Kart, Castlevania, and Banjo Kazooie.
PS1- Castlevania SOTN, FF8, Bushido Blade, Dark Stone, Diablo, Coolboarders, and MDK.
Sega Genesis- Sonic(series), MK, Pitfall The Mayan Adventure, Ghouls 'N Ghosts, and Tinhead.
I have spent more time on the Atari 2600, Colecovision, and NES than I have any other system that I have ever played.