What are your favourite moments when Optimus Prime and Megatron have had to team up to stop a threat bigger than the Autobot-Decepticon war? G1 – The episode COUNTDOWN TO EXTINCTION, where Optimus uses Megatron’s gun mode to fire a critical generator into space, and THE CORE, where Megatron assists the Autobots in stopping a berserker Devastator. Granted, both of these times were due to putting down Starscream (who was taken back into Con ranks), but it was still great to see. G2 – Optimus and Megatron team up to stop Jhiaxus, and later Starscream when he possesses a planet. It’s got some great moments between the two, where Optimus meditates on how they could have been friends, and how Megatron assists Optimus a few times (out of affection – after all, he enjoys the rapport they have and wouldn't want to lose it). PRIME – Megatron and Optimus join forces to take down Unicron inside him. A great reversal from G1, where Megatron says he is commanded by nobody and actively works with Optimus against the Chaos Bringer. DOTM – This sadly only really occurred in the comic adaptation, where the two actively work together to take down Sentinel Prime. If only the film could have had this happen... Anyone else?
Novel!Movie Prime and Megs fighting against Sentinel. That was pretty awesome, particuarly towards the end where Megs slides Sentinel's legs out from under him, giving Optimus time to level the acid blaster.
IDW, on the few instances they weren't at eachother's throats, mostly because they both, originally, shared ideals. There was a time they wanted the same thing, and we actually saw it develop instead of just being told about it for a change.
Armada. That was a true alliance. Hell even after Unicron was defeated for the next 10 years there was peace as the Autobots and Decepticon agreed to live together...off course Alpha Q had to ruin that.
Seconding this reason. Galvatron entrusted his troops to Prime as he then sacrificed himself to save everything.