So you know it is /b/ not [b]. A lil lesson for you (as I have gone onto 4chan times before) /b/ is one of the topics to select, its the random subject.
Back to OP: Technically since optimus is a prime he should only be able to be defeated by other primes, not megatron. But since Transformers 2 effed that logic up I would go with Optimus still. He's over 9000!!11!
If the two robots were picking are different ones from the movie (no actual story) I would go with megatron. He is just way cooler, and transforms in to an alien ship (sort of)! Optimus is also cool, but it's a truck!