Using SDCC, Keil is probably the best, but there is ofcourse a big price difference.

I don't know if one should have 'proper documentation' for a compiler. When you run your (sdcc) compiler without arguments, all switches are explained. And most of us DO know how to read the source.
I don't want to start a flame war, but when you've seen one (C) compiler, you've seen them all.

Eric