The three teams go at it, complete with their respective Sixths (and we'll throw in Ninjor for the Alien Rangers even though they never actually appeared on screen together).
Round 1: Base forms. No Battlizers or Super forms.
Round 2: Everyone is resurrected for the Megazord battle with their entire arsenals and fleets (including the Battleborgs, Ninjor, Falconzord, and Titanus [yes, I know] for the Alien Rangers, and the respective auxiliary and carrier zords for the other teams)
Bonus:
Same battles, but replaced with their Super Sentai counterparts: Kakuranger vs. Hurricanger vs. Ninninger
Battle is in and around Angel Grove, since the city seems to have every type of environment for each element - forests, deserts, the waterfront, downtown, the tundra, and even a goddamn volcano. What a town, eh?