A good lens to me is a fast lens (maximum apperture of 2.8 or below), it can be a zoom or a prime lens but must be sharp at most one stop down from maximum apperture. I don't mind light fall-off in the corners since it provides a natural vignette, distortion doesn't bother me either, just as long as it's fast focusing and doesn't hunt much in AF.

I find this mostly in prime lenses like the Sigma 20mm 1.8, Canon 35mm F2, 50mm 1.8 and lastly 85mm 1.8.