The 24-105 f/4.
At a glance the f/4 figure will seem like a downgrade from the f/2.8 of the 17-55, but the short answer is the full frame sensor will more than compensate for the smaller max aperture.
Neuro will probably be along in a mo to explain exactly why. The technical details escape me
Haha, good call. Neuro's thoughts are exactly why I want to move to FF. I want an L class standard lens with IS, and an L class ultrawide, thats not too horrible on distortion. Thinking of 17-40 (when not an UWA will be a standard on 60d backup) and a 24-105L. Waiting on some reviews of the 6d before I choose a body. Id love a 5DMkIII but that won't be happening :-/
I would go with a 5D II over the 6D. It's cheaper and still an excellent