Neeko's Ult is Overloaded
Massive AoE damage, AoE slow during prep, AoE stun on execution, a massive AP scaling shield, and it is uninterruptible unless she's killed. On top of which, if she starts it while disguised, you won't see its preparation even if you know it's her.
It's like Riot threw everything they could think of into it. The only thing it doesn't have is a complimentary gap closer.
This is among the most disgustingly broken ultimates in the game.
I say one of the most because a rank 3 Kassadin ultimate remains unsurpassed to this day, but that's another topic of discussion.