Generally speaking, regardles of incarnation, Optimus Prime's alt mode is going to be some type of truck. There's not quite the same level of unity as far as Megatron's modes go. He's been quite a few different things through the stroll through the many universes of Transformers. Which do you prefer. Are you one who's a G1 purist and love the classic gun mode. Are you one who has a liking for one of his tank modoes. Perhaps a Cybertronian assalt vehicle or ship is more to your liking. Maybe something else entirely. What do you think?