I shoot the Canon 24-105 f/4 L IS-USM as my "main lens" on my Canon 5D...its image quality is far from "amazing". "Good, but not great" is a more-accurate description of its image quality and imaging performance. And that ,"Good, but not great" IQ of the 24-105-L is based on its use on a mere 12.8 megapixel, full-frame camera. I shudder to think what kind of lens flaws a higher-resolution body would reveal. I use the 5D and 24-105 for my weekend snaps and fishing trips. For "serious" work, the 24-105 L is easily bettered by numerous other lenses that I own.
I actually prefer a tele-zoom for MUCH of my personal work, so if I had to have only ONE lens, it would be a camera-maker's 70-200mm f/2.8 zoom, with stabilizer.