Let's throw a wrench in the works: ACTION MASTER Megatron versus G1 Galvatron.
Both are upgraded forms of G1 Megatron, but which upgrade is more powerful? Plus the whole sanity thing. (Galvatron: "I traded sanity for POWER in the plasma pools of Thrull!" / Megatron: "Oh yeah? Well I just took a bath in Nucleon, and I still think I'm Ratchet!")
This brings me to another point: G1 is not a single universe, but several. The Galvatron from the Japanese "Headmasters" cartoon is not a future version of Marvel UK Megatron, TFTM Galvatron is not a future version of Dreamwave G1 Megatron, Marvel US Galvatron is not a future version of G1 cartoon Megatron, G1 toy Galvatron is not a future version of War for Cybertron Megatron, etc. Therefore, since Transformers have demonstrated the ability to travel from one reality to another, G1 Megatron and G1 Galvatron could fight--to the death, even--without causing any kind of time paradox.
So why not try this one too: TFTM Galvatron versus season 3 Galvatron. (Maybe one of them's from the American cartoon and one of them's from the Japanese cartoon, if you want to really nitpick it.)
Also, Marvel Megatron versus cartoon Megatron, and cartoon Galvatron versus Marvel Galvatron.
One shall stand; one shall fall; and the one who stands shall help the one who falls to stand once again.
--Rhymus (also known as STARS Commando 539, also known as transit)