Dont' touch the Yamahaha, too trendy. I had one for a short period, and dumped it quickly for many reasons.

The F4i is really a nice bike, great power delivery, probably of the four Japanese manufacturers, the most street ridable.
I'm riding a GSXR1000 right now, and it's ideal, but back when I rode 600s, the Suzuki was a real racer. They always have been, sort of the slightly less refined but balls to the wall power.
For comfort though, definately go with the Kawasaki, those things are like Cadillacs on two wheels.
If I was in the market for a 600, my order of preference, for me would be:
Suzuki with Honda a close second. Yamaha third a fairly distant third with Kawasaki a close fourth.