I always love to see the concept art. It's a shame that they've never done an art book for any of the Transformers films. But thankfully many of the artists have shared their work online...
Vitaly Bulgarov
Warren Manser Age of Extinction, Dark of the Moon
Joel Chang
Robert Simons
John Park part 2
Wesley Burt
Ben Procter
Josh Nizzi
Ryan Church
Steve Jung and some of his AOE art
Steven Messing
Massive Black
CGSociety article on Alex Jaeger
Paul Ozzimo
George Hull
Feng Zhu
Fausto De Martini