Kawasaki Ninja 250
Kawasaki Ninja 500r/EX500
Suzuki GS500E
There are quite a few others. I'd recommend buying used, put on some serious miles, and then make it a point to upgrade afterward. It's the skill of the rider, not the beauty of the bike that earns true respect afterall.

These are just my humble recommendations. It's really up to you in the end. Just be careful, have fun, and don't make cagers hate riders in the process. h: