I saw the OS .28 XZ this weekend and its an absolute ROCKET! Very impressive!
There are many to choose from but depending on the price range, its up in the air!
I have heard the Jammin mills are pretty good... They have a .21 and a .28
You can go all the way up to the $400+ price range with the supersweet GRP engines...
I agree the C6 is very nice!