It's fun to get to level 50 on Galaga or play Pac-Man for an hour, but not if you are playing two players... (unless the 2nd player needs some sleep or likes to read). What's more fun is two player games where you go mano a mano with the other player. Not cooperatively shooting, or taking turns, but where your goal is to destroy the other player and have your victory reduce them to a sobbing sack of goo.
So the latest "Hall of Fame" on my blog contains games have already been Games of the Day that are especially fun because of the two player competition. Some of them are VERY simple games but tons of fun nonetheless. Obviously, EVERY good Vs. fighting game would fall into this hall of fame, but remember this is only for games that have already been GOTDs on my blog and I try to make those more obscure games, so you won't see any Street Fighters or Mortal Kombats here.
http://mamezach.blogspot.com/