Reverse Polarity, Black Hole, Chronosphere, Overgrowth, Ravage, Rain Of Chaos and Earthshaker's 3 stuns synergize really well with pretty much all other AoE ultis as well as with each other
(just it's not a good idea for the whole team to pick only heroes with ultis like those, because the opponents, provided they're smart enough, will push all 3 lanes at once, in which case you won't have all your ultimates at the same time, at the same place, in order to defend properly)