I've already got a couple of threads on Optimus and Megatron about which voice actors have played the characters the best, but more and more TF bots are getting compared so its better to consolidate to 1 thread.
check out the audio clips and see images of the actors who played them and judge for yourself.
for the new bots
Starscream - Chris Latta (the original screechy voice, though Tom Kenny does a great impersonation)
Jazz - Scatman Crothers (he just bumps out Phil LaMarr coz he brings out more of the jive)
Prowl - Jeff Bennett (Prowl really hasn't stood out as much as other bots, but the Animated version gives him more depth)