My favourite tank Megatron mould would be 1994's Hero Megatron - even if he is purple with a fluro orange air pump! He's like a smaller version of 1993 G2 Megatron, only with an articulated robot mode (making that toy a pioneer of articulated Transformer action figures).

Ultimate Battle Megatron does have nicer colours, but mouldwise it's not as neat as Hero Megatron - I mean, in context of their vintage. UB Megatron is a fairly ordinary figure by 2006 standards (the lack of a right hand and the way the gun-hand falls out so easily is disappointing, though the rest of the toy is fairly nice), whereas Hero Megatron was just an outstanding piece of engineering by 1994 standards -- among the first of a new breed of articulated TF action figures that we take for granted today.

And as weird as it looks, the air pump launcher gimmick is pretty fun. (although the pump and hose can sometimes be cumbersome) THOOMP!