Nekketsu Kouha Kunio-Kun is a port of Technos' arcade classic that invented the brawler genre. Despite coming out in 1987, this game shows an astounding amount of brawler refinement: it has difficult enemies that require strategy and skill to complete, boss fights, vehicle sections, and a huge moveset that's still more advanced than most other NES beat 'em ups. It also has a catchy rockabilly soundtrack.
This is based off the arcade game that invented the brawler genre.
Kunio would go on to star on over ten games, from the RPG/brawler River City Ransom to sports games like Nintendo World Cup on the NES.
Kunio is the best. It'll undoubtedly turn off most players at first, but once you learn how its combat works it's pure joy. The unbelievably huge movesets were way ahead of its time, and the face-punch attack is infinitely satisfying. The graphics are awful, but the music's super catchy and memorable. The labyrinth is dubious, though.