I've mostly played Space Invaders (it has coop on 2600), Asteroids (weird diagonal moving asteroids, so different from arcade), Pole Position, Battlezone (pure movement and dodging vs two tanks at once, no obstacles), and Robot Tank (like Battlezone but enhanced) on 2600.

oh, and HERO, although I actually played the SG-1000 version of that more; still a fucking fantastic game