Master Megatron.
I always preferred the way Takara handled that. Armada Megatron upgraded to Super Megatron; then to Superlink Galvatron, who upgraded to Galvatron General; then Galaxy Force Master Megatron into Master Galvatron. I like the use of "Master" as the prefix for their names, implying that they are the perfected or mastered forms of the characters.
Speaking of, while Energon was an okay use of the term as a series name given that the substance did play a decent part, and Cybertron wasn't a terrible choice given that reviving Cybertron/Primus was the goal of the story, Micron Legend, Superlink, and Galaxy Force give you a much better idea of what's going on. Hasbro could have easily translated the title to Transformers: Powerlinx for Energon, which would have been better. I guess that with the number of G1 homages in that series, they wanted a name that would call back to the series.