The insanely detailed alien robots that smash their way around our screens doing Michael Bay's bidding all start with some very cool concept work. Here is some for Transformers 3 which shows the evolution of the work of the first two films. Clearly it is a complex thing to create the CG robots, putting all this energy into the concept pieces is also quite epic.
Josh Nizzi's site has a heap of his work on TF3 and here are a few that inspire me (heaps more on the site):
[Josh Nizzi Transformers 3 concept art via his site]
Warren Manser's site gives us another view through his Transformers 3 concept pieces, again heaps on his site and a couple of choice picks here:
[Warren Manser Transformers 3 concept art via his site]