Which special feature does the attack titan have? As far as I understand, there are 9 different types of Titan Power. One can harden his skin, other can become armored, other is a giant, other can gather other titans, etc. Which special power does the Attack Titan have?
As far as we know, immense source of strength and willpower.

Keep in mind that its not only one shifter that can harden their skin. I.e Eren does the same to close the Wall Maria hole. 

Origin: When the Progenitor Titan (aka Ymir Fritz) died, her soul was split into 9 parts. But this does not mean that each of those parts had only one special feature. We have seen all 9 titans and some abilities seem hilarious compared to others (for example "The Jaw" Titan vs "The Beast" Titan. 

Thus, maybe each of the 9 shifters has multiple abilities which take a lot of training. But this does not mean that the shifters are different from each other based on one ability.