In celebration of the upcoming movie, I decided to rank the worst to best Megatron.
Energon: This guy was nothing but a one-dimensional character who is a former image to what he was in Armada. The character derailment in this series is far worse then Beast Machines. His army only made up of a few soldiers who are nothing but reprogram versions of the Armada Decepticons. I blame him for the deaths of 6 billion Decepticons as they rammed themselves in the energon grid and got fed to Unicron.
RID: He did nothing but sit on his tailpipe till he became Galvatron. He was stupid enough to believe a Locomotive is a WMD and made a fool of himself in front of humans when he plan to blow up a dam. 
Scourge should of been the main villain.
Cybertron: Not much to say other than Sith powers. His ultimate plan to rewrite the universe is cool however.
G1 post-movie Galvatron: This guy is insane and would gladly hurt his troops over the smallest things. He didn't know what he is doing. For an 80s villain, I'm actually scarred of this guy, you have no idea what he can do, he can kill anyone just because he fills like it and seems really powerful with his blasts.
I have a theory that he got cured at the end of Season 3 from the Matrix but the season 4 writers must of overlook it and went back to insane Galvatron.
G1 movie Galvatron: It's G1 Megatron but more cruel. Sadly, he went insane.
Early-Early Energon: Seeing him bust out of Unicron's walls, makes Scorpinok his slave and owned the Autobots bases in seconds made him awsome. Sadly, other than the 500 episode special, this show was only good around 10 episodes.
Movie: Little to be said about this version due to the fact he was only in the last 30 minutes. Unlike Venom, The built-up to Megatron being release was cool, busting out of his ice prison with a mace, ripping Jazz into 2 and beating the slag out of Optimus is cool and don't feel badly rushed. Mr. Smith did a good job wasn't a bad misscast like that 70s guy.
Armada: I would rate this Pokemon collector lower but I feel this version of Megatron had more depth. Feels lost when his greatest nemesis over many centuries pass away, feels sorry for Starscream's sacrifice, and sacrifice himself to save every transformer on cybertron. Sadly, the writers of Energon forgot about this and Energon Megatron wanted Unicron to devour the same planet he wanted to save 10 years ago. What the slag?
G1: He may of been an 80s villain but when I see him, I see a huge war commander who is keeping his troops together.
Beast Machines: Character Derailment sucks because he have a hate for organics that came out of nowhere. Compair to most of the BM and Energon cast, he didn't suffer from derailment that bad. It's neat to see Megatron in control of all of Cybertron, commanding the whole world in his mind. He almost became a god and became a threat as bad as Unicron.
Animated: This version is a mix between G1, BW and the movie. Unlike G1, Megatron would kill traitors without a 2nd thought. G1 Megs would let Starscream live after he stabs him in the back 32745 times.
Beast Wars: This version has the best personalty with his smarts, catchphrases, and humor. His ultimate plan was to nuke Optimus Prime's head so he can create an alternate timeline where the Decepticons win the Great War. Like Xanatos, you sees the upside of things even when things are looking down. This Megatron alone has nearly destroy time and space, conquer Cybertron in hours, and almost became a god. The only other Megatron that went this far Cybertron Megs by nearly rewriting the universe.