The 24-105mm f4 L IS lens is a great walk around lens and spends most of its time on my camera. The kit price of a 5D Mk2 and 24-105 is less than the sum of the parts, so well worth going for if that is the lens for you. An alternative, but heavier and more expensive lens is the 24 - 70mm f2.8 L. As others suggest the 70 - 200mm f4 L IS is a fantastic lens and a great choice if you want the short-medium telephoto range. At the wider end is the 17 - 40mm f4 L lens.
Another, less expensive and truly excellent lens, well up to L standards is the 50mm f1.4 lens. Nice in low light with a big wide aperture (so matches the 5D Mk2 capability in that respect). The choice, as ever, will depend on what you need the lens(es) for.
All are great lenses but you will find shortcomings if you really look hard enough and/or read some forum posts both here and elsewhere. I have found them excellent if all I want is to take photographs and print them even up to very large sizes.