I am amazed how many times Kawasaki's are mentioned. I don't see them so regularly where i live. But anyways, all of the Japanese 600cc are a solid start, but you might also want to look into Ducati, if you care for V2's.
Also depends on your height imho, i would recommend you test drive at least all of the Japanese bikes to get a feeling for how the seating position is. There are large differences between the manufacturers.
I originally wanted to buy a Fireblade, but the seating position was not right for me. I never thought about bying a Ninja, but when i test drove i just felt at home.
TLDR: Go test drive and find the bike that suits you