Grace Under Pressure and Moving Pictures are probably 2 of my favorites done by him. Really, it's amazing to see how many different artists he's worked with, and how many different album covers he is responsible for.
However, I gotta agree with Kotowboy in that he really seems to have gotten lazy or at least doesn't pay attention to the details like he used to. Perhaps this is because of the use/ease of Photoshop, taking on too much work, getting older or something else. But the quality of his artwork isn't what it used to be. Ideas are stellar, but the execution of them is lacking, especially on most of DT's albums.
As for why he's not involved with The Astonishing, I asked JP that, and his response was because the 3D modeling and stuff is not what Hugh does. If he did, you can be sure that he'd have done the artwork for TA, and I wouldn't be surprised to see them use Hugh again on the next album.
Not to get off topic, but JP did say that they originally had gotten in contact with some Russian artist (didn't mention his name), but that the ideas he submitted to the band were too dark for what they were going for, and so they kept looking, and discovered Jie Ma.