I think all of these were mentioned above already, but my favorites (in no particular order) are:
- Combatribes (SNES)
- Return of Double Dragon (SFC)
- Battletoads & Double Dragon (SNES)
- The Peace Keepers (SNES)
- Streets of Rage 2 (Gen)
If it were for arcade titles I'd say: VENDETTA
One beat-em-up that I personally find very disappointing and had high expectations of are Knights of the Round (SNES). The arcade version is slightly better though but still.. Also, Final Fight 3 is great fun but WAY to easy even on hard-mode.