Which champion has the most powerful ultimate buff?

The only ult that matters.

With AP, it's Nasus. But I'm the only one who builds AP on him. In that sense I'd say Yasuo because it gives his team plenty of time to set up and finish what he doesn't. Most surviving enemies drop out of Yasuo's ult with 50% or less of their health, and when his team is all set up and ready to jump on them, it's often a death sentence. Granted, that assumes he hits all five.

Are we talking just stats? A champion that you do not have on there gives you the most stats.

I'd say for offense stats only

Defense it's 5 seconds of invulnerability.

Alistar ult is honestly really fantastic and he gets mega fucking tanky except he’s a support so he can’t benefit as greatly

either , , , , or (passive effect that is)