Wow, I'd completely forgotten about Bob Joles.
While I'm all for his
Origins voice, Joles would be pretty cool. I love Steve Blum, but for me, he's a little
too iconic. I'd see Mewtwo and think,
Why does Mewtwo sound like Spike Spiegel?
If I HAD to choose an actor to perform spoken lines for Mewtwo, I'd pick Micheal McConnohie. I've loved his skill as and actor since hearing him portray Margulis in Xenosaga.
Here's an example:
http://youtu.be/SLxe2knWI8Q?t=1m30s
Can't you just hear Mewtwo growling out something like: "I thought you better than this."