ef-s 17-55 f.2.8 IS is my walk-around lens on my 7D. Great images; sharp, fast, quick-focusing lens, relatively solid physically. Expensive: I shudder to think what an "L" version of this lens would cost, but we get what we pay for, mostly.
Otherwise, I would have to take the easy way out again and select the 70=-200 f/2.8 L IS (I) for iq, and zoom range. Not so much for walking around with, but sharp, fast, quick focusing lens that gets along well with my kenko 1.4 tele.
I was favorably impressed by the 24-105L f/4.0 IS on my recent evaluation 6D as a full frame fave. The 6D's superior noise-handling ability easily offsets the lens' slower aperture.