I'd say Ironhide: Ironhide: Why are we fighting to save the humans, they are a primitive and violent race. *Optimus Prime does his speech* Ratchet: Ironhide we should leave this planet. Ironhide: That's not what Optimus would want. Ironhide: You feelin lucky punk. Bad Mojo. The parents are very irritating can I take them out. In the second film with no Optimus to stop him he could have obliterated the NEST forces, but restrained himself despite angrily grieving Prime's death and not liking the fact he has guns being pointed at him. DOTM Ironhide encourages Optimus to speak with Mearing despite Prime being angry at them. And surprising throws down his weapons for a ceasefire with the Dreads. I repeat the trigger happy Ironhide threw down his weapons and resolved thing peacefully with the Decepticons.
Starscream, he was always sniveling and sucking up to Megatron for most of his scenes. But by the end of the third movie he got real quiet.
Believe it or not I'd say bumblebee. By the third movie he was more mature according to bay I think that he mentioned it awhile ago then tlk came out he was like the one in charge yes he said I dont fuck with you to hopskins character.
Most characters in the Bayverse (or the vast majority of Transformers fiction for that matter) don't get any development and change through the course of the movie(s), but I also think there are some better examples of character development than given credit. So with that said, I'm going to say Optimus (AoE), Megatron (over multiple movies), Hound (AoE), Joshua Joyce, and Wheelie (RotF).
In order to have a "best" of something, that thing has to actually exist first. There isn't any character development in the movies so there can't be a best character development... Well not for the Transformers anyway... they weren't really treated like characters in those movies, they're more like set pieces that talk. It's one of the main reasons I hate those movies. There really isn't that much difference between an alien robot disguised as a car and an actual car because they aren't treated like characters. Sam even literally still refers to Bumblebee as his car rather than his friend. So yeah, they're treated more like objects than characters. I think Eleanor from the movie "Gone in 60 Seconds" was more of a character than any of the Transformers. In case you've never seen that movie and think that Eleanore might be a person, it's a customized Mustang. I actually didn't know that movie was a remake till I was looking it up just now to figure out exactly what kind of car Eleanore was but there's actually two. They're both customized Mustangs but the original was a 1971 Ford Sportsroof redressed as a 1973... and I'm confused how anyone can tell it was a 71 if it was redressed to look like a 73 but I know nothing about cars and literally just reading what it says on the wiki. The one I'm more familiar with is a Shelby GT500. Though now I know why they didn't just call the car Shelby since the original version was a different model. I've never seen the original movie so not sure how it was portrayed there but Nickolas Cage sure treated that car like it was a real person only ever referring to it as she, her, or Eleanor. Other characters in that movie even made fun of him for it since he was literally the only person treating that damn car like it was a person. His girlfriend at once point even complains that he treats that car better than he does her. That said... I think the best character development in the movies is actually Simmons who goes from being an antagonist in the first movie to a some what hesitant ally in RotF, to actually being a true ally and friend every other time we've seen him after that. I don't mind humans also getting some character development but is it too much to ask to actually get some Transformers in my Transformers movies? They aren't just set pieces, they're the title characters. They're suppose to be where the main focus is. Instead the focus is almost always on the humans with the Transformers filling supporting roles. That's like making a Superman movie and then spending the entire movie focused on Lois Lang, Parry White, and Jimmy Olsen. Superman might show up in the background at some point or when he directly interacts with those characters but never actually focusing on him as a character in his own movie... which actually might be interesting for a one off but I wouldn't want to see six entire films written that way.
I know people are used to dismissing them out-of-hand, but honestly, the prize probably goes to the Twins. It's hard to tell under their incessant jabbering, but they actually go through something of a character arc during RotF. When things are going smoothly for the Autobots at the beginning of the film, they behave like star-struck teenagers, focusing less on their training and more on gushing about how cool the big guns are. They get psyched up when their granted brand spanking-new alt-modes, jazzed that they've been promoted to the big leagues. Then things go to hell. Optimus Prime dies and the team is scattered, and the Twins begin acting noticeably more twitchy and nervous now that they don't have the big guys at their back, and without any true combat experience to fall back on. Then, they stumble into their encounter with Devastator, and finally experience firsthand what personal loss means, and what they're fighting to protect, and in that instance, they man the fuck up and bring the heat.