Who would it be?

I'd have to say kennen... those ults.
Plus you wouldn't have to go ap on all 5.

Lulu
:>

5 leonas full AP
AoE AoE AoE, tanky as **** in general, so much CC

Malzahar, just imagine their entire team suppressed with MV and his puddle.

Karthus obv.

Hit level 6 --> GG

^
Singed
/gg karthus and all others

Garen!
Garen everywhere

i think teemo because come on mushrooms everywhere

i think it should be teemo because if you had mushrooms everywhere it would be domination

definitely gonna be karthus. enemy team would have to stack magic resist items at lv 1 and go full mres runes/tank masteries just to survive a synchronized ulting. and karthus's could just build normal
not to mention tanks and meleers of all forms would just be obliterated upon engaging with 5 karthus aoes active

second place id probably give to akali for being bursty with decent sustain and what i assume would be share-able shrouds. also she does 2 forms of damage so harder to counter than something like say 5 graves