Corey Burton is my choice for absolute greatest. He did Spike and Shockwave in G1 AND Animated (Spike had a cameo in one episode), and also was a FANTASTIC Megatron in Animated.

David Kaye's also been around, I mean, he's the only guy to have voiced both a Megatron and an Optimus.

And Lance Henriksen's Lockdown is one of the greatest single voices ever.