Oh man this one is pretty hard to choose...
Leaning towards Inverse Doubles at the moment. Inverse is probably my favorite battle spot special format, and I bet doubles would throw a MASSIVE twist on that! For example, imagine how Landorus-T would have to deal with actually be weak to Earthquake now, or how Aegislash is suddenly super terrible now, or Mega Kangaskhan's crazy new Fighting resist.
Anything Goes triples sounds NUTS though. before you see "OH GOD NOT DARKRAI", note that Im positive people would bring Klefki! Imagine all those Arceus though good lord...
No Evlutions sounds sorta interesting, though I imagine that would be another metagame where Mega Kangaskhan rules the roost. Maybe people would use interesting single stage stuff like Throh though, since no evolutions limits the playing field a LOT.
I imagine no mega evos would have some strong similarties to gen 5 GBU, except there would be plenty of Aegislash and Talonflame of course.