It comes from RotS novelization. When Anakin says ‘This is the end for you, my Master’, Kenobi replies ‘I have heard it many times, Anakin. I never thought that I was going to hear it from you’ (or at least something similar, I have read the book only once).
I remember finding that very powerful and describing Kenobi’s sadness more than anything (especially considering that the entire book is focused on the relation between those two).
Of course, Lucas had to replace it with ‘Only a Sith deals in absolutes’, ‘From my point of view, the Jedi are evil’ and the ‘best’ one ‘If you are not with me, then you are my enemy’.
Another thing is that some good bits seem to have come by accident. For example, in the movie (and especially in the novelization) it is quite clear that Sidious is allowing Windu to win in order to force Anakin to turn to the dark side (he goes within a second from ‘I am weak’ to ‘Unlimited Power’). Probably Stover saw the movie and made it even more blatant in the book that Sid threw the fight. Then comes Lucas and says ‘Na, Windu just overpowered Sidious’. :facepalm