I would guess a standard zoom, something like a 28-70 or so is the best place to begin. Depending on budget, there are a number of options from Canon and third party suppliers, Sigma and Tamron.
The best quality option is probably the Canon 24-70 2.8L. This comes in at around £1000. It is very good, very heavy, and expensive. I think it's too heavy as an everyday lens, so i'd consider the next option,the 24-105L.
Canon's 24-105 F4 L. This is more like £900, but can often be found approaching £700 as a white boxed model. This lens is very very good, works wide open with little loss of image quality, and is an IS lens so you get the benefits of easier holdability in low light. It is about as good a general purpose lens as has been made.
Next down from Canon is the 24-85 3.5-4.5 USM. This is a good price (more like £300), and is light and generally pretty decent. It's not that fast, not that long, but for 95% of photography is a good bet.
From third party suppliers, I use the Tamron 28-75 2.8. This is a good lens wide open, very sharp from F4 and on, and is relatively light for it's maximum aperture. Problems with this lens are the build quality is a bit plasticky, and the autofocus is an old style motor rather than a USM. I've had great results out of my copy. Tamron also offer a 7 year warranty which should keep you going (I presume this is still the case).
If you are interested in primes rather than zooms, the canon 50/1.4, 50/1.8 and 85/1.8 are all very good lenses, and if you want standard/portrait length these are good bets.
So really it does depend on budget, but then, I guess you aren't surprised at this.
Cesare
To give you an idea, I have the following lenses at my disposal for canon full frame SLR/DSLR use:
Zooms:
Canon 17-40 F4L
Tamron 28-75 F2.8
Canon 70-200 IS F4 L
Primes:
35/2, 50/1.8, 85/1.8, 135/2.8
My zooms are pretty comprehensive and i've nothing to complain about here. The primes could do with getting upgraded to more decent models at some point, but they each cost >£1000 to replace, so i keep shying away from this...