NES? Super Mario, of course.
For SNES? Chrono Trigger.
PC? Starcraft. People STILL play that damn game on line.
N64? Perfect Dark.
PS1? Bushido Blade.
PS2? Star Ocean: Until the End of Time.
GCN? Super Smash Bros.
X Box? Halo 2 (online)
Gameboy? Kirby's Dream Land
Nintendo DS? Screw...
KOTOR. KOTOR 2. Halo. Halo 2. Panzer Dragoon Orta (because everyone else downplays it). Fable. Ninja Gaiden. MechAssault: Lone Wolf. Prince of Persia. Need I name more?
But not only are they a hell of a lot more portable than a TV and a DVD player, they cost around $15-$20. And there's a suprisingly large selection of them. AND, the battery lasts just about as long as gameplay battery life: 6 hours. Unless, of course, you're stupid and set the volume to max and...