The Co-op in this game...

[SmashFinale]

Honestly, in my opinion, really is one of the best co-op experiences in a game out there. It far outpaces sonic 2 or 3 in terms of co-op play. I always thought the best co-op gameplay is one where you dont NEED to be sharing a screen to play together. Like i recently played this game with my best friend and sometimes i would complete the level without him and vice versa. If either of us were stuck we would use the partner call mechanic to get out of that area and save each other from a headache. I honestly wish more game would follow the example this game set for us.
The only other game with a better co-op than this would be Kirby and the Amazing Mirror imo.