Magnus. Like what Starscreamer95 said, it would be easier. And he was one of my favorite characters in the G1 movie.
We could have Omega Supreme on the good guys side and Unicron or Trypticon on the bad guys side. Imagine what an EPIC battle would be? [and most likely would be in the space].
Ultra Magnus But that's comming from the Magnus fan inside of me, also Omega Supreme is to difficult to do justice.
I know that the tanker truck's identity is confirmed, but I'd have loved to see Ultra Magnus as the truck, especially with that deco. Be a subtle yet nice homage to his line "I'm just a soldier", and not to mention it'd be a fresh take on the character's design. More or less, I'd be satisfied with just a toy of Ultra Magnus, even if he never appears in the film. Just a toy-only character like Breakaway, or maybe Hasbro will still try to cash in on some movie-style toys after the movie itself has done its thing, like ALLSPARK POWER for MOVIE 1 and HUNT FOR THE DECEPTICONS and [kinda/sorta] REVEAL THE SHIELD for ROTF.
Where's the 'neither' button? As much as I like the characters, I'd rather have another character(s) get more love, more obscure characters which haven't shown up in a while. *cough* Wheeljack *cough*
And Trypticon, and Reflector, and Sixshot, and Optimal Optimus, and Dinobot, and a pony and a tricycle... Personally, I'd vote neither as well. I'd rather see interesting characters and plotlines over a fan-wanky 'star-studded' bot lineup anyday.