?
Jayce
even the bad ones can 1v5 your whole team...genuinely surprised he's not in this vote...also yorick? only time I see him in aram is when I roll him and have no rerolls...he's not that annoying either..
Xerath
Nidalee? Spears only come from one direction. Stay behind your tank and if you don't have one, then quit standing in one spot.
Katarina is easily the most annoying of the four.
If you ever happen against this combo:

You'll know what pain is.
As someone who plays a shitload of ARAM, I can say with ease it's Katarina, Yi, or Karthus. Karthus is really really painfully OP in ARAM. Brand is pretty stupid too.
But yeah. Kat. shivers
SONA has been broken in aram forever.
Nidalee by far. Even worse is Headhunter Nidalee.
Katarina can be shut down with hard CC or a silence. Nidalee just gets fed off her spears and then snowballs.
Anyway, thats of the one's listed. Personally, I think
is the most annoying ARAM champ to play against. Every single one of her skills is irritating to deal with.
Off-topic, but that cosplayer is gorgeous.
What's a Yorick?
Karthus is way worse than any of those.
Xerath and Teemo are honorable mentions as well. How on Earth did they not make the list?
How is yorick annoying besides giving the enemy 5g every few seconds?
?
The number of pentakills from mushrooms I've seen is astonishing.
I personally hate playing against
most on ARAM. If I get hit by a Nidalee spear, I can hang back and regen it up, (or side step next time), but if I get hit by ONE Blitz hook, there is NO do-over, because he will CC lock you while his team mauls you, usually before your team even realizes what happened. You may say "just get better at dodging his hook" but on a crowded ARAM map, even if you dodge half of them, he is going to land at least 3 out of 5 on someone on your team, even if he is bad at Blitz, and every grab is an almost guaranteed kill for his team until about 20 minutes in.
None? There are way worse champs...Xerath with his range etc. But Nidalee unless you have a really bad team comp. Because Kat can be stunned easily if one has an Annie, TF, Jax, etc.
Edit: And as far as spears being dodgeable, they are often not the only things flying at you, not like 1 v 1 in ARAM so you can just watch for spears.
This is the most annoying champ in aram, you know if you draw him as a pick.
No AoE damage, No poke, just basic attacks. Is so annoying to play trundle in ARAM. xD
None of the above.

Mostly Sona though. A good Sona can otherwise carry a pretty bad ARAM team.
I'd say Lulu, Jayce, Master Fap and Karthass.

I voted Nidalee but you should have expanded this to include several others with #1 being Yi. Talk about a cheap champ...
AP Caitlyn ftw.
New Zilean, They all just sit in bushes and throw an endless barrage of bombs at your team.
then you get 50% by xerath from out of vision range
spamming that nightmarish laugh.
Huh.. I came in here expecting to see either
or 
Those turrets and bombs can be extremely annoying lol
Sona.
Also, an average Kog'Maw is pretty scary. Just constantly lobs Rs and Es at you.