If they're adults, stay away from Simpsons or TMNT. Single button side scrollers with no end are boring after awhile.
Definately classix such as Donkey Kong, PacMan (we prefer Ms. Pacman in our house), 1941, Galaga, etc.
For 2player, Killer Instinct or Killer Instinct 2, any of the Double Dragons, etc. As brought up before, definitely Track 'N Field...if your CP is stout. It takes a lot of beating during this one.
For 2 player coop, definitely Bubble Bobble.
AJ