I'm not sure that making a buying decision on a new body should, at this stage of DSLR development, be based on individual criteria. You have to look at the whole package. Low light shooting is getting very good (compared to 5yrs ago) no matter what you choose. If you are a wedding photographer, its a no brainer. You have to go with full frame. Best, and most used by pro wedding guys, is the 5D or 5D mk II.
If you are an amateur, like me, there are many good choices. If you shoot a lot of sports and "need" 6+ FPS. The choices are limited to 7D, 40D, 50D, or 1Ds. I would stay away from the xxD series. Shutter, and shutter button problems!
As far as a complete package, I dont think you can beat the 7D. 100% viewfinder, 8FPS, great autofocus performance, respectable low light performance, very good build, pro level shutter, whether sealing, HD video, very good picture quality, reasonable price for the level of camera features. I have the 7D - I love it.
The rebel series is also a good way to go if you are looking for a somewhat "disposable" camera after a few years use, with the view to buying a "keeper" sometime in the future.