Ask anybody who has shot with Canon gear for awhile, the EF 24-70 f/2.8l lens is a must have lens for anybody serious looking for a zoom that can handle relatively low light. It's construction is amazing .. I once had champagne spilled on my copy, and my 40D, and I had no problems what so ever (aside from some stickiness).
As a walk-a-bout lens, it excels in quality from wide angle to mid zoom. On the 40D it has decent reach, but on a full frame sensor (like the 5D), it might not have the reach you desire. In that case the 24-105 is a better choice ... But, you sacrifice the f/2.8 ability, which can be very important at times!
Color and contrast from this lens is excellent for a zoom lens. Stopped down, the lens only gets better.
What could be better than this zoom lens? Prime lenses. A dream set of primes would be a 24, 35, 50, and 85. The only problem is you'd have to switch lenses all the time!