This rather obscure game of the classic Sonic era was apparently a concept for Sonic 4 in the early stages, but instead became a game featuring Knuckles and many other new characters with a unique gameplay mechanic.  The game has you tethered to another character, and you use each other for momentum and attacks.  Although rare, some of the music in this game inspired later Sonic efforts with its happy but somehow sad tones.  Tracks like "Door into Summer" and "Evening Star" are still discussed in forums on music theory.