'Kay so...
1. Optimus stomped the Fallen who himself no-sold his own re-entry of 1.29 kilotons of TNT
2. Optimus easily curbstomps 3 opponents of similar power level and possessing similar abilities, and is a force to be reckoned with melee wise, maneuverability is only a problem in flight, but in melee combat that's a non-issue for Optimus.
3. Iron Man definitely has better flight capabilities and maneuverability, a lot more weaponry in his arsenal, more versatility, a hilariously good AI system, is slightly stronger at 1.76 kilotons of TNT, and much like Optimus has also fought against opponents with similar powers multiple times, but melee combat is straight up his biggest weakness since he pales in comparison to Cap's ferocity.
4. Iron Man has 7-B lasers as his ultimate trump card, but unfortunately Optimus knows about them here.
I'm inclined to take Prime on this one, mid-high diff.