I have some suggestions, keeping esthetics and transformation in mind. Mind you, with Megatron you automatically set yourself open to Galvatron repaints.
With that:Robots in Disguise Megatron
No, not the Prime one, the 6- or 10-changer from 2001. The Hasbro versions both use the same mold (the Devil Gigatron retool), so both can achieve 10 modes at least. Black or White, your choice. I would be careful with the gold chrome paint as it has corroded to a nasty green on mine.Energon Megatron
Simple transformation yet looks awesome in both modes. He's modeled more after G1 Galvatron (to the point he is named Galvatron in Japan through the entire line), but that shouldn't stop from at least checking him out. His little tank is cute too.Cybertron Galvatron
The only instance in which I prefer the repaint over the original, simple because he has G1 Megatron's colors. A tad complicated, but his race car mode is unique among his forms (even with RiD Megatron), the jet mode is... decent but the robot mode screams power.Henkei/United Megatron
Get him, get him NOW! He is, and will be the only regular-sized Megatron with a gun mode we'll ever get. Don't look into he fact he's got a backpack, and is based on a NERF gun (not to mention the US version being colored as one), but the Japanese colors make a BIG difference. United should be easier to come by, but I prefer the Henkei one. The chrome on the scope does chip off, so be careful.Revenge of the Fallen Voyager Megatron
The Leader one is good I'll admit, but the Voyager has better proportions, easier transformation and is more screen-accurate in design... just not the colors. He was planned on being repainted, twice in total, but neither came to pass. So the sea foam green is your only option.Dark of the Moon Voyager Megatron
I had a hard time recommending this guy, but the vehicle mode and the use of the tarp is worth nothing. Also interesting is that despite the robot mode being made of the whole vehicle, trailer and all, the vehicle mode has a (limited) point of articulation at where the hitch would be. The robot mode is... plain, but the tarp adds character in concealing the molded damage to his face.Animated Leader Megatron
Possibly the closest in G1 Megatron's design, and just plain cool looking especially with the Fusion Cannon and his two swords. 'Nuff said.