What is the most annoying ult ?

If you're squishy, later into the game Q+R = grey screen. - he's pretty much the same story. Spins for 2 seconds, then yells "Demencia!" and you're dead.

And obviously

I really hate Kayn's ult. Alot of times I'll be able to 1v2 a gank, but never when there is a Kayn. You think you're gonna kill him then he presses R and comes back with a ton of health restored.

pretty much solely because of the cd, like yeah lemme just grab my 1000 dmg laser from halfway across the map every 20 seconds

All for the same reason. You have to beat them with atleast 40% of your hp remaining or you simply can’t beat them.

Not sure why, but ult seems to be magnetized to me from across the map. She could fire it in the entirely opposite direction and id still find a way to get hit by it.

Allied Bard is by far my biggest annoyance. Saved so many people that I was about to delete.

For strictly enemy... I'd probably say Zed. Not only does it basically double his damage due to the proc at the end, but it also allows him to dodge my Q or R, leaving me completely unable to win the fight.

Mordekaiser. But if you really want the essence of an annoying ult, teemo