Dark Of The Moon is definitely the most action-packed of the three films. Besides that, it's extremely funny, and mixed in the moon landing of Apollo 11 with all the conspiracy you could expect in Transformers.
The cast was good. Shia LaBeouf was extremely funny, Rosie Huntington-Whitely was damn sexy, John Turturro just cracked me up, Kevin Dunn and July White were hilariously awkward, John Malkovich was something special, and, well, there's a lot I can say about Ken Joeng. Also, the transformers had new, roboty sh*t going down, like worms and shit... It's a must see.