Megatron in whatever form has always been the best leader of the decepticons. In one of the comics in the late 80s the decepticons were lead by Ratbat. Ratbat????

Come on even for G1 standards that was hard to swallow as he was a cassette tape tied to Soundwave. There was even a time with Shockwave in charge in the comics and I hated that as well.
Starscream would make an ok leader but still extremely pale in comparison to Megatron.
As for the Beast Wars version of Megatron I thought he was cool and was up to par especially when you realized he was actually at first doing the bidding of the original G1 Megatron. Just have to say loved that they had a refomatted Ravage in the Beast Wars series even if it was just for a little over 1 episode.
As for the Bay Megatron I think he was very savage and raw in the 2007 movie which fit as he was frozen for thousands of years just to be discovered by humans then was kept on ice and use to start the computer age, a lot of rage would fester in me if I was used that way.
The ROTF Megatron was an progression of the 2007 one as it seems he realizes it take more than raw force to win. But as I said his raw force in 2007 movie was completely understandable and IMHO real.
Megs toying with Sam, being underhanded in fighting Prime in the forrest (by having two of his decepticons in the fight rather than one on one), blaming Starscream for his own failures, deciding it is time to reveal the tranformer race to the world and finally crying for Starscream to help him when he got owned was all definatley G1 Megatron. I loved it!

t:
Really loved when Megatron got his metal butt kicked and his face half blown off his first response was to yell out "Starscream!"