Well, for EF-S bodies, I guess it would be the Canon EF17-55 f/2.8 IS... but that's kind of a lot of money to pay for a dead-end mount (as in you can't upgrade to a full-frame body). With the full-frame 5D pushing around $2500 now, I wouldn't bet against full-frame. I think a better choice would be the EF24-70mm f/2.8L or the EF24-105 f/4L IS.
I'd have to agree with Cliff, EF-S lenses are risky if you expect to move up to a full-frame camera which I expect you will once you outgrow the 20D and prices come down on the new full-frame bodies like the 5D. I would definitely recommend the 24-70 myself. It's a very versatile lens, albeit a bit on the heavy side.
