OK, the IMDb webpage for Energon doesn't have the voice actor who did Megatron's voice. To me, it sounds like Sean Connery. Or at least Darryl Hammond's Celebrity Jeopardy version of Sean Connery. If it is indeed Sir Sean, then it is certainly some of his finest work!
shareDavid Kaye did the voice of Megatron/Galvatron in Energon, he also did Megatron in Beats Wars, Beast Machines, Armada and Cybertron. He also voices Optimus Prime in Transformers Animated
-- COOOBRAAAA! --
David Kaye based his Megatron voice on Sean Connery's, he once stated that in an interview, but his voice in this series and the rest of the Unicron Trilogy was just bad due to the horrible dubbing job they did on the show.
I think he's far better as Megatron in Beast Wars and Beast Machines and as Optimus Prime in Animated, infact he's so good in Animated I wouldn't have even guessed it was the same person who had voice Megatron in the previous Transformers cartoons.
Oh and Animated is 100000 times better than this show, if you haven't already, check it out, it really goes out of it's way to pay tribute to the original show.