If you could Have only One

[Colin]

But, to answer your question, for a 1.6x crop body it would be the EF-S 17-55mm f/2.8 IS. On a FF body it would be the EF 24-70mm f/2.8L.

I think I'd have to agree with your selections, neuro.
<div style="CLEAR: both"]</div>

I think so too... the 24-105 beats out the 24-70 in terms of IS and reach, and I think it has better control of CA outside of the focal plane, and I really like the 24-105, and don't want to give it up if I can keep it, but... the 24-70, even if it is a clunky bastard with less zoom range, just makes me happy in ways the 24-105 doesn't (as much). To be honest, I wouldn't bitch about having either.

[cxr]

24-105 f4 IS USM