Not exactly united, but Transformers Prime/Beast Hunters ended with Megatron offering peace to Optimus and exiling himself, because through Unicron's will, he felt the oppression that he sought to impose on others.
"That's right, Simmons. The war is over. We get to go home. In fact, you're the big hero. We're gonna hold a big parade in your honor. I'll get to drive the float, and Grif here is in charge of confetti!"
Oh maaaan... oh man, oh man, oh man! I'm trying so hard not to cry! Call me stupid, but I first saw this pic in english class, and I alomst started to cry right there... It's so amazing! It tell perfectly about "Till All Are One"! Everytime I look at it I feel like I'm gonna cry... specially at optimus and Megatron... and StarScream and SkyFire... just so many feelings! Love it! :') :')