Depends on what all you are looking for.
I borrowed a friends 24-70 2.8 while in Hawaii a few months ago to try is as a good walk around lens. I had been debating between it and the 17-55/2.8 IS EF-S. I really liked it, but never really needed 2.8 ever and could have used a little more zoom (i have a 1.6 crop 400d). I think I have decided to get the 24-105/4 L as a walk around. You can find them usually on a deal on craigslist from guys who have bought a 5d kit and don't need the lens.
The other lenses I use the most are (in no order)
After getting the 24-105, I plan to get the 100/2.8 macro to complete most everything I can see I would need most often.