I'm not sure if anybody even played this game before, but it's a old Saturn game... Guardian Heroes (The original one). That's one of the best, if not the best, beat-em-up that I've ever played.
Anyone who's played the recent Bleach DS fighting games (No, it's not crap!) would have a good idea to expect from that game actually. Dual planes, mad air combos, multiple players,air combos (!), awesome fighting engine, parries, juggles, Supers, level ups (It's a partial RPG), multiplayer co-op, infinite combos...
Dammit, does anyone still have a working Sega Saturn & Guardian Heroes? All of a sudden I really feel like playing it again...
On the topic of You missed out a few really cool co-op games as well (Off my head) :
1. Macross Arcade
2. Cadillacs & Dinosaurs
3. Aliens vs. Predator
4. D&D : Shadow over Mysteria (Come on, who doesn't remember doing the QCF + jump sliding around to hoard the gold/treasures?)
5. ALL TIME FAV: Raiden
There's no Gunstar Heroes or Streets of Rage on any list I've seen so far.