Mine is:
Master Yi,
Shaco,
Nidalee
Lux.

These guys all dominate ARAM.

Ziggs / Zilean / Blitz / Yi / Fiddle / Nid

you could argue that a few of them are not so good depending on the enemy comp but.. yeah they are still annoying lol

sincerely even soraka/sona can be ridiculous annoying depending on your team/ enemy team comp... dem sustain! :O

I don't find many champs annoying to fight against, but I do find it annoying to play on the same team as a Darius or an AP Yi, mostly because I have seen allies with those champions stand aside and watch me fight for 5-10 seconds just waiting until the enemy is low enough for them to come in and grab the killing blow themselves.

Yi/Nid/Lux are just heroes that should encourage you to buy MR. An Aegis on the team, and one solid MR item (plus Merc treads vs. lux) renders them pretty impotent... but then many people don't really pay enough attention to alter their build based on the match-up.

Master Yi

A decent Wukong is a very scary thing. He can tank, carry, initiate and escape all with the one kit. It also helps that, with a Black Cleaver, you can drop their entire teams armor by 25% with your ultimate. He is ridiculous in ARAM.

He also fits in to just about any team composition out there, so it's never bad to have him.

A good zilean is such a pain to deal with. The bombs are a great poke, speeding up certain champs has great results especially if they don't have a gap closer, and the ultimate obviously complicates things.

Caitlyn. If you can't outpush enemies and Caitlyn isn't dumb, you can say good bye to your towers.

Janna.

Teemo. Shrooms everywere

Since when was shaco even remotely good on aram.