I love beat'em-ups! All of my best memories of bonding with my brother were through these games. I think most of the best ones have been mentioned already, e.g. Double Dragon, Streets of Rage, Final Fight, TMNT, Simpsons, X-Men, River City Ransom, even lesser known gems like Cadillacs & Dinosaurs, Alien vs. Predator, Knights of the Round, and Guardian Heroes.
I'm surprised no one's mentioned Golden Axe and Splatterhouse!
I was also an unabashed fan of Captain Commando, the Punisher, Rushing Beat, and Sengoku.