When Optimus shot Megatron's face using his own weapon in ROTF at Egypt, and when Optimus slashed Megatron's face using his axe in DOTM? And I realised now Optimus Prime makes lots of noise when fighting, lol.
It took quite a few replays and analysis, but I think I got it: "RaaaHUUUUUUUGHNaaaraaaaaaaahhhhGRUUUUUUUUUUUUUHKshoguuuuuuuuRAAAAAAAH!!!!"
I think he was babbling because half his head was blown up. That sort of happens when you only have half a mouth, y'know?
0:33 "Your on FIREEE!" See Megatron can compliment Optimus, even when he's about to get his head ripped off.
I heard random babbling, but my cousin heard "RETAAA-" I think after the forest fight, Megs needs to return some names to Prime...
im pretty sure he is saying "My eye!" I heard in ROTF and its reused when he gets the axe slammed in his face in DOTM.