Personally I think primes are the way to go, unless you are actually using the zoom in a shot, leave those alone.
I use old Nikkor lenses on the 5Dmk2. Stunning results. I have a 24/2.0, 35/1.4, 50/1.4, 80/1.8 and 135/2.8. They are not as sharp as the Canon Primes, but it has that dreamy...