Superion, the Aerialbot combiner Computron, the Technobot combiner Defensor, the Protectobot combiner Raiden, the Trainbot combiner Landfill, the Build Team combiner I'd go for Superion myself. A plane combiner is something the Autobots badly need, with their notable lack of air support. I'd put Raiden as runner-up, cause I like trains and think they're cool.
In theory: Superion Based on what we've seen done to TF designs via the movies and particularly what happened to Devastator: N/A
To be honest, I'm not a big fan of the Autobot combiners, so I'd rather not see any of them in the Movies.
Aerialbots/Superion would work very nicely, I think. The perfect opportunity to introduce them came in ROTF, but as usual...
Superion if any. But like TRANS+CRAZY said, I'd rather they not have any. Makes the Decepticons seem that bit more powerful.
I'm partial to Defensor. He's always been my favorite Autobot combiner and I'd love to see what he'd look like in the Movieverse. With that being said I would not oppose to a Superion combiner. It'd be really neat to see them going toe to toe with Starscream ... errr if he was still alive that is.