Warbler said:

I was just thinking . . . remember how the Emperor told Luke that he had sent an entire legion of his best troops to deal with rebels on Endor? How could there be any best troops, when they are all clones?

To bring the EU into it (which is now not totally true, since The Clone Wars is basically second-level canon and therefore not really EU) ARC Troopers and Clone Commandos are trained much more differently and more extensively than is a regular clone trooper.

To bring more EU into it, by the time of ROTJ, a lot of clones had indeed been replaced by human recruits and clones of other Imperial soldiers (done in a different and less reliable way and trained less extensively than even regular clones, focusing more on a doctrine of "overwhelming force" rather than fewer highly-trained soldiers; this was done as Jango Fett's DNA samples had by this time deteriorated and the Emperor no longer trusted Kamino's cloners due to their uprising), this was due to the rapid aging of the clones because of their accelerated growth and therefore deterioration; however, at the time of the Battle of Endor, the legion of troops sent to Endor was the 501st legion, which was still a mostly-Jango cloned legion, making them "the best".