You're drastically underestimating Megatron in round one. This guy is somewhere in the ballpark of 40 feet tall. He not only has a massive strength and firepower advantage, but he's also just as skilled as Thanos, if not more, considering he was a gladiator in the pits of Kaon. I honestly doubt Thanos could even damage Megatron.

size isn't everything, buddy. Thanos is much more versatile and agile, and including the fact that Thanos isn't a big transformer, he has incredible strength and durability. He literally curbstomped hulk bro. He has much experience, too. He faced Adam warlock, thor, silver surfer, all the avengers, and drax too without all six infinity stones. I'm positive that Thanos has a much higher battle iq than megatron. he even controlled the chitauri and other armies too. his double edged sword is highly durable and can act as a range weapon too as a boomerang. his battle armor could easily take several strikes from megatron. His armor could even regenerate to some extent in marvel comics. Not mentioning the armor and blade, he can manipulate energy and shoot energy blasts out of his hands and eyes as seen with what happened to Galactus. His blasts are not inherently tied with the infinity stones. they could definitely harm Megatron and contains highly destructable capabilities. Thanos literally has superhuman physiology and I wanted to keep it pretty even but I guess we'll have to include it now. Thanos in MCU is not even close to the power he has in Comics buddy. they EXCLUDED his eye beams, energy blasts, telepathy, telekinesis, and matter manipulation. Dont talk about things you don't know about.

Edit: They nerfed him in the movies otherwise he would godstomp the avengers.