i feel like i always get hit/interrupted/killed when i pop nunu ult

Anytime I see a nunu coming into a fight I save my interrupt for him. It prevents me from using the skill, sure, but I'd rather have a wasted regular skill than a wasted ultimate. That being said, Cleanse + Nunu = lol.

Nasus' ult when he was first introduced scaled with AP and did ridiculous dmg!

Ryze's Desperate Power ult can be incredible powerful if the enemy is bunched up for a lane push. Late game you can get overload to do 1k+ dmg (from mp+AP) and flux doing I believe 300-400 a bounce (and -15 MR to each target). Run in, Ult, Flux, Overload and if the enemy is really bunched up, you will deal incredible damage to all of them because of the 65% AoE, follow this with a rune prison to help cooldowns, and you can do it again before ult wears off, if the enemy isn't all dead or running for the hills at this point.

Tyrandameres does the most damage.

Ultimates in and of themselves alone:
Fiddlesticks
Tryndamere
Veigar
New Ryze
Taric

Ultimates for teams:
Amumu
Nunu
Gangplank
Malphite
Taric

The rest scale from "meh" to "not bad situationally" to "good enough"

I'm really surprised Highlander has not been mentioned yet.

It increases Yi's attack speed, movement speed, makes him immune to all slowing effects AND if he kills a champ while under Highlander ALL of his CD's refresh.

What other ult has 4 attributes to it?

Twitch's ult out of stealth...gg towers