well, that there is a tough question. I would also say it really depends on how serious you are in photography, and how important speed is. I mean... if you want a do it all lens, you could get something like a 28-200 (or a 50-500)(ish) it's not real fast, but it does cover a large range and can be quite useful.
If you're concerned about speed of the lens, but also want a zoom range, I would probably suggest say a 17-55 2.8is or perhaps say a 24-70L or something like that (you could go longer as well if you already have the kit if you wanted to.)
If you're primarily concerned about the sharpness and are ok with using your feet to zoom you could get a prime or two... A couple of good ones are obviously the 50mm 1.4 (or 1.8 if you want to go quite inexpensive) or maybe the 85 1.8 (or 1.2 which is a tad pricier)
It really depends on what you want to do with it.