The first movie was good -- it captured the spirit of G1 cartoon quite well.
The second movie was just too "try hard," the first movie was simpler and worked better. The 2nd movie got too convoluted for its own good and got tied up in its own inconsistent mess.
The third movie was better than the second, but still too convoluted and not enough plot consistency.

The entire trilogy lacks substance and real character driven stories. So many of the Transformers, especially the Decepticons are NOT treated as characters but as set pieces... like giant robots coming to get-choo!

Good characters are ones that the audience empathises with and emotionally invests in; we have to CARE about them. Okay, I was upset when they killed Ironhide -- that was well done. And when Bumblebee was about to be executed, that was a really emotionally tense scene -- my heart was pounding and I was at the edge of my seat during that moment! So well done there too. But I didn't really care about the deaths of any other Transformer in the entire trilogy. The death of Optimus Prime in ROTF was done well too -- but as a long time fan that has no effect on me since we all know that Hasbro would never allow anyone to kill Optimus Prime permanently. But maybe that was a tense moment for non fan audiences. But other than that... meh. Megatron died twice, both times I didn't really care either way.

Look at The Dark Knight Rises... I was able to empathise with all the main characters in that film at some stage at some level -- even Bane. A good story is one where you give a crap about the characters and what happens to them. I find that the Bay movies don't develop the Transformers enough as characters that makes audiences have that emotional connection with them. Instead it's mostly watching them do cool action fight stuff and go "WWWHHHHHOOOOOOOAAAAA!!" Look at Yoda... his best moments wasn't when he was jumping around with a light sabre during the Clone Wars, but when he was a decrepit hermit living in a swamp teaching Luke Skywalker how to be a Jedi. I mean, if you'd never seen the Original Trilogy before, and you were watching the Prequels for the first time -- if Yoda had been killed in Attack of the Clones or Revenge of the Sith, I don't think audiences would've really cared -- not much more than say the deaths of any other Jedi in those movies (Qui Gon Jinn, Mace Windu, Aayla Secura, Kit Fisto, Even Piel, Coleman Trebor, Plo Koon, Ki Adi Mundi and the rest of the Jedi who were killed under Order 66). But when we see Yoda dying in Return of the Jedi, and Luke goes, "Master Yoda, you can't die..." <--- that was really emotional! No fancy CG effects and explosions... just a muppet with a throw blanket.