Honda CBR600
Kawasaki Ninja 600

Both are great learning bikes and very forgiving to new riders but can also be pushed very hard as you advance. I suggest the older carb'd models although the newer injected models are fine too they just have a bit faster throttle response.

I wouldn't reccomend a smaller bike because you will outgrow it quickly and if you can show me anyone who can honestly outgrow a 600 as far as ability to outperform the bike then I will bolish your nuts myself...600 class is the performer class when it comes to racing.