I've picked Nick (and not just because he is a mate) narrowly ahead of E.J. Su. Nick makes the individual characters look distinctive, giving the Transformers facial features beyond the normal generic face. His art has a great energy to it and this is conveyed in each panel. He is also able to turn his hand to different environments and they all look good.
E.J is a close second to him, but Nick is the main man.
It's easy to say Nick as he's a great guy and he posts here, but he's also top notch on the art. Not a fan of EJ Su, not a huge fan of Don F but he has his moments. I am a fan of Guido. Can't say the others have made much of an impact on me either way.
Spotlight: Galvatron would probably be my choice for best drawn IDW issue.
EJ Su and Nick Roche are neck-and-neck for me. Both have come on leaps and bounds, both in terms of composing covers as well as interior art. Honourable mention for Mark Bright's one issue. I'd like to see him on art again sometime.
Don has improved in leaps and bounds since his Dreamwave days and has definitely gotten better. Guido does have a nice clean style which works really well. I like E.J.s technical skills as every redesign he has done so far looks plausible, which not every artist can manage. He has a firm grasp of storytelling but sometimes I find the art is a bit too static, hence why Nick get's the Turnbull seal of approval.
I quite like the variety of art styles: more akin to the Marvel era than the housestyle Dreamwave had. While not all the art is to my taste, I do like how all the various IDW TF books look quite different and there's something for everyone. The only art style that really irriates is Alex Milne's. He has decent design sense and has knocked out a few decent covers, but I find much of his sequential art hard to follow.
I do agree that Don Fig's storytelling skils are much improved.