The voice of Optimus Prime is...

[chorrocks]

I'm clearly late on this, as it was announced some time ago at Comic-Con. But the voice of Optimus Prime is...

Peter Cullen.

The original voice actor. As well it should be.

Now all the need is Frank Welker to pick up most of the rest. (Sadly, Chris Latta and Scatman Crothers are no longer with us.)