As mentioned it really depends on what you want to take shots of. For a zoom, you can't go wrong with the 70-200 F4L or the 50-250 IS.
The primes offer a way to shoot in low light without needing a flash due to the large aperture. The 50 1.8 is good but has problems focusing in low light and is also quite slow to do so.... they periodically fall apart too but can take very sharp photos. If you want to spend more, there is the Sigma 30 f1.4 or the Sigma/ Canon 50 f1.4 too.
On the other end of the scale, the wide angle lenses are fun to play with, sigma 10-20 and Tokina 12-24 will let you get creative The canon 10-22 is more expensive.
You could always just get an improvement on your kit lens with the more versatile 17-55 f2.8 IS. It has the best of all worlds, offering a wide aperture and IS. On the down side, it does not have much range and is still pricey for a non L. It is incredibly sharp and worth the cash though.
It is a slippery road you are walking down... once your start, it is hard to stop!