Ninja 250: very light/agile, forgiving, easy to corner and ride. Top speed about 100mph, 0-60 in around 5.5 seconds, which is quicker than most cars. A lot of fun to "beat on," meaning you can really wind out the first few gears even around town.
Ninja 500 and Suzuki GS500(like Smitty mentioned). Both also lighter motorycles, just with more power than the 250. These are definitely quick enough to get you in trouble, but still toned down enough where you're not going to be accidentally doing wheelies.
All of the above are also very reliable bikes, and can be picked up used at decent prices. The good thing is when you're ready to move on, many times you can sell them for what you paid for them(if bought used), or close to. Plus there's always a big market for these bikes, so they're easy to sell.