In the first Stadium game, Mewtwo was the final boss, especially with how overpowered Psychic was in that gen. In the second Stadium game, pic related was the final boss, using the three main Ubers that existed at the time (as Mew and Celebi were just secret event Pokemon I believe).

In a theoretical Stadium 3, who would be the final boss and what would their team be? Would it be full of legendaries? Would it be full of cheap tricks and annoying strategies? Would it be OU: The Team? Would there be a Mega Charizard X and would the character's name be Alain? He was indirectly referenced in one of the games after all