You know, I thought that was way too long as well, until I actually read it. Very good and concise post.
Let me get this out of the way 1st: I grew up on G1, especially the Marvel comics. The very 1st issue of Marvel comics I ever read was issue #13, titled "Shooting Star" and guess who the main (and only) Transformer character was? I've loved the Megatron character ever since. In the last 30-odd years since I've been a fan, I've loathed the character of Optimus Prime in almost all incarnations, except Optimus Primal from
Beast Wars (who is technically a separate character, but still represents the Optimus of the series) and Fire Convoy from
Car Robots. All other Optimus Primes make me want to gag. So, my emotional response is that my preferred Megatron alt mode is the Walther P-38. No stock, no longer barrel. Just the handgun. For Galvatron, same thing. Big tripod cannon with an orange noodle for a barrel.
Now, for my rational response.
I think that Megatron, being the leader, should have an alt mode that best represents his personality, as well as one that is practical. (Which the gun is NOT, for reasons
Kurona already mentioned.) For me, this would be the jet. The tank is a satisfactory alt mode, but it's not very practical, due to its lack of speed and maneuverability. Of course, it makes up for it in firepower and near-invulnerability, which is preferable, especially in hand-to-hand combat in robot mode, as we saw in the G2 comics. But for the purposes of being a supreme leader of an army, having to be all over the battle-field to make sure the fight is going your way, being a jet is preferable to be. It makes Megatron more mobile and if he has enough firepower he can fight better as well. The only downside is he's more vulnerable to attacks, especially when he's flying. So I guess both modes have their pros and cons. But if I
had to choose, I would choose the jet mode. He had it in
Prime, in
Revenge of The Fallen, where technically he was a triple changer, and in
Robots in Disguise, where it was 1 of 10 modes he had.
However, my favorite
rational alt mode for Megatron is his truck mode from
Dark of The Moon. It was big, ugly, powerful and menacing, just like Megatron himself. It was faster than a tank, yet probably still had the same destructive power, or close to it.
I did think Master Megatron's alt mode of a race car from
Galaxy Force was a unique take on him, even though he was out of scale. Then I saw a digibash/customization of the toy into Rodimus's colors, and it just looked badass.
Galvatron? Like I said, the G1 tripod cannon will always be the iconic alt mode for me, but I think for him the jet mode is also the most fitting. His volatile personality requires that he be mobile and agile, and a tank or a cannon just can't do that. He has to be a plane in order to satisfy his constantly changing moods and needs. For this, my preferred Galvatron is the one from
Super-Link.