In TF1 Megatron was a beast, he was the villain of the movie. It was supposed to show how menacing and powerful he is. Obviously he was powerful in the movie and destroyed anything in his path, including Optimus. Reason being I think Optimus couldn't go full on because of the many humans around them. Look at the highway battle, Optimus killed Bonecrusher so fast, and there weren't nearly as many humans around. In TF2 there were threads saying he wad significantly weaker because of the parts made from a weaker decepticon. But if you notice he had a more powerful alt vehicle mode, half tank/jet and took more explosions and missles than he did in TF1. Optimus took on Megatron, Starscream, and Grindor all at once WITHOUT the Matrix, and no humans but Sam around. The reason Megatron killed him was because he was looking for Sam and Megatron struck him from behind. In the end he beat Megatron like a tin can and only significantly damaged part of his head, I doubt his brain was damaged. Nothing that couldn't be repaired. In TF3 Megatron was just on the sideline. The major villain was Sentinal not Megatron. After letting Sentinal do all his work, pretty sure after the plan would be a success Megatron would have disposed of Sentinal one way or another anyway. He also beat the living daylights out of Sentinal in the end, he didn't looj no where as weak as many claim he was. Although he did it behind Sentinal's back, I'm sure he could have fought if he wanted. Also Optimus caught him off guard so it wasn't a full on battle. Though I don't know why Megatron switched from his jet/tank form to a truck because he wasn't significantly damaged in TF2. Don't forget on Cybertron all the Transformers fought and have been damaged/repaired all the time so it's not the first time he.was damaged. In G1 Optimus beat Megatron most of the time so it's not a suprise to see Megatron lose.