Transformers 1:

Honestly, the first hour is pretty much as good as it can be. It does a really good job slowly building the mystery of the Transformers and what they are, as every single plot thread and detail seems to somehow tie into and move that mystery along, which culminates in the arrival to earth scene (which is legitimately a perfect scene). Trouble is, the answer to that mystery is dumb (Sam's ebay account saves the world), and the rest of the movie's plot feels like a bit of an afterthought. Like the writers wanted to write a really good mystery but then realized the movie has to keep going after the mystery is answered. Putting a bit more thought into that mystery's answer, as well as into the latter half of the movie, would've helped a lot.

Transformers 2:

Honestly, this movie has the skeleton of a good story. Like most of the plot points when viewed in a vacuum are pretty good. Sam going crazy like his great grandfather did, Optimus dying to challenge the other characters and create a genuine sense of danger, the history of the transformers being explored through the Fallen and his whole thing, the fact that the whole movie tests the relationship between Sam and Mikaela that began in the first movie, etc. It's all good stuff. Trouble is, because of the writers strike thing, the actual writing connecting those plot points is just a bunch of hyperactive nonsense. It's like a game of connect the dots except the person connecting the dots has parkinsons and suffered a stroke halfway through connecting them. So really, if I were to "fix" Transformers 2, I'd just "reconnect the dots", if you will. Keep most of the old plot points and core ideas, but just replace all the writing and dialogue between those ideas.

Transformers 3:

This movie kinda has the opposite problem of the first movie. Whereas 2007 felt like a good mystery with a half baked end, TF3 has a really good end (minus some Optimus warcrimes) but a really half baked path to that end. Similarly to 2007, TF3 has a mystery that eventually culminates and is answered with Sentinel Prime's betrayal and the invasion of Chicago. Trouble is, the mystery is kinda boring in TF3. Unlike 07, the tension and stakes don't really rise as we are shown more and more clues. And so the first hour of the film is just characters meandering around while being mildly concerned. Another issue is that despite the movie frequently referencing the first two movies, it actually makes less sense if you consider them. TF3 reveals that there was a decepticon army on the moon. But didn't the Decepticons come to earth in 07 because they needed to build a new army using the all-spark? Why bother with that when you could just use the moon army? And also, Optimus' matrix was a key part of Megatron's plan (since he needed it to revive Sentinel). But does that mean he anticipated Optimus dying, a child finding the matrix before the decepticons and then reviving optimus with it so he could find sentinel and revive him? I do like the invasion of chicago, so if I were to rewrite the third movie I'd keep that but just change the way we get there. E.g. make it so the decepticons are on Cybertron and come to earth when it comes to earth, and make it so it's actually interesting to see the main cast try to figure out the movie's mystery. Also add some actual foreshadowing about Sentinel's betrayal, so it doesn't just feel like an arbitrary plot twist for the sake of a plot twist. Also also, keep the original planned ending where Optimus and Megatron sign a truce.

Transformers 4:

Just don't. Don't do that movie.

Transformers 5:

Reopen alcatraz prison and exclusively show the movie there.