Ultimate, as they control three Pokemon at a time that can be switched out with each other. First introduced in Super Smash Bros.
The Pokemon Trainer is a unique fighter within the roster of Super Smash Bros. However, one modder decided it shouldn't be another Pokemon that is made playable, but a trainer. One franchise that many fans have speculated would represent the final fighter is the Pokemon franchise. Ultimate's final character, many fans have theorized about who could possibly take the spot. With the impending arrival of Super Smash Bros.
The roster has continued to expand with the game's two Fighters Pass DLCs, brining in characters from all walks of gaming like Sephiroth from Final Fantasy 7 and Minecraft's Steve. Ultimate is already one of the largest in gaming, having many different characters and franchises represented.