Honestly not a big fan of the genre, but one in particular I loved that I can't believe hasn't been mentioned...
Castle Crashers! How has no one else mentioned it? Usually Beat 'em ups are pretty simple, but Castle Crashers has like a billion characters who all play differently, tons of equipment choices for weapons as well as bonus animal friends that give perks and a leveling system to specialize in different forms of combat. It is also just great on aesthetic with so much humor and interesting enemies/bosses.
All these things make Castle Crashers better to play on your own then average, but I feel like it's just a genre that suffers unless played multiplayer.