Okay, well...Ive sat on the R6, GSX-R (600cc), ZX-6R, and CBR F4.
Ive riden all of the bikes except for the ZX-6R.

Of course I am going to say the R6, because I own one.
But, I really think what should determine your decision is what exactly you are looking for in a bike.

My second bike was a 2000 GSX-R 600. I bought it simply because it was being sold at an insanely cheap price. It was a nice bike, and I had a lot of fun on it...but when I bought my R6, I felt like I just went on a diet and dropped a shit load of weight.
The R6 handles much better (in my opinion). The R6 is lighter than the GSXR and I think it has a more comfortable seating position. On my GSXR I had to lean foward alittle farther to reach the handle bars. In general I feel more comfortable on my R6 because I feel that I have more control of it.

As far as the CBR...I rode my friends 1999 CBR F4. It was okay, but for some reason I felt like I was on my first bike again (1997 Katana) . It just didnt feel as sporty as the GSXR or the R6.

If you read any of the 1998-2002 Cycleworld or Superbike reviews, when they compare the 600cc class, in general the Ninja comes in 4th (not sure exactly why since ive never riden it), the CBR in 3rd, and the GSXR and R6 usually battle it out for 1st/2nd. In most reviews, the R6 comes out on top...due to its great all around performance (handling, acceleration, braking, streetablilty, etc.).

Im a big fan of handling because when it comes to choosing any of these four bikes, speed and power are really minor factors....all four bikes are insanely faster than cars and will get you from 0 to dead in all about the same time (give or take a tenth of a second or so)...its handling, rider comfort and streetablility that's important. Whats the sense of all that power when you cant feel comfortable with it?

Damn I sound like some rep from yamaha....

I think you should try to test drive all four bikes if possible before buying.