Cave Spiders

They are in my opinion the toughest mob in 1.8.

General spider update: no longer 2 by 2, they're now 1 by 1. They can fit into any sized gap in any direction.

Poison takes out most of your life, cannot kill you, but that's easily changed with a mob's attack. There is nothing you can do once you're poisoned, you just have to outlast it. (as far as I know)

They often hide above you behind a 1 block high cover. I've found myself being pounced on by 3 at once, I had no idea where they came from.

I swear their attacks reach further than spiders used to as well.

Which mob do you think is the toughest?