Best crop lenses depends upon your subject matter, shooting style, and budget. I shoot lots of grade school sports, both outside and in poorly lit gyms. I also shoot candids at a variety of events and figure skating. (Plus the typical tourist stuff.) Lighting is often a challenge, but the 7D sensor with a sharp f2.8 lens does a great job in these situations.
Granted, the full-frame big brothers can handle low light better than the 7D, but my background is with push processing Tri-X 400 to 1200 and then jumping to the Rebel XT. Everythinng's relative. To me, the low light capability of the 7D is fantastic. I wouldn't give up on low-light with current Canon crop sensors just because full frame (and some Nikons) have an edge.
I'm shooting a 60D (same sensor as the 7D) with an EF-S 17-55 f2.8 USM IS and a 7D with a EF 70-200 f2.8L USM IS-II. They cover everything I need.
Prior to these lenses, I resorted to the Nifty Fifty 1.8 and the 35 f2.0. Both great for low light and the 35 is particularly sharp on crop bodies. For some events I often try to use just the 35, but I always end up going back to the 17-55 for it's IS. (The 35 is great for close-up work, less than a foot from the focal plane.)
I have other lenses -- EF-S 17-85 4-5.6 IS, EF-S 18-135 3.5-5.6 IS, and the EF 70-300 4-5.6 IS. But, I'm always prefering the speed and sharpness of the 2.8 zooms over the range that these other lenses offer. Cropping the 70-200L Mark II gives me better images than the 70-300 at full zoom. And the ability to isolate my subjects at 2.8 is a huge advantage.
Of course, this means I often carry two bodies (I hate changing lenses and losing shots). The 60D has been a good match for the 17-55 with its flip out screen. I'm most likely to want high or low angle shots with this lens over the 70-200. But, the 7D with the 70-200L Mark II is dream to shoot with for everything from small venue sports to candids from across the room. If your budget can handle the 70-200 2.8L Mark II, don't hesitate -- it will likely outlive a few bodies.
