The Energon Pub

...I'd say the third. I envision them fighting at the middle of the seocnd act and shout belief systems and rants at each other. The third movie would be perfect.

I'd say save it for TF 3. I think we all want Starscream vs. Megatron in TF 2 where Starscream gets beaten within an inch of his life and then Megatron spares him because he's too valued by Megatron to get rid of. Only Galvatron has the willpower to get rid of Starscream. Still, in TF3, Prime has gotta beat Megatron fair and square. Prime got no justice in the movie when it came to fighting Megatron and the Primes has usually been on par with the Megatrons.

i would actually think they should bring him back in tf2 rematch prime at the end and gets injured mortally,but not destroyed.
then he turns into galvatron in tf3 in the final battle

Agreed Roadbuster, Prime can't hold back on his "brother" anymore the next time they fight.