My experience is defined by the time I've spent on these bikes:
1995 Kawasaki Ninja 250R (EX250)
My first motorcycle - it was a pretty easy choice too. There seemed to be few small displacement, sporty bikes out there in 2001. The Ninja 250 was plentiful and affordable, so the decision was made for me before I even began shopping. This trusty steed would survive my fear, erratic use, and frequent neglect. It wasn't until around 2004 that I began using it regularly because it bought me a free ticket to the HOV lanes in Northern VA.
2004 Honda Shadow VLX (VT600C)
Commuting on a motorbike was my thing, and the Ninja 250 had a few problems I was too lazy to fix. The idea of a comfortable, high torque cruiser seemed awesome to me at the time, so I forked over much more than I should have for a brandy-new Honda Shadow VLX. I learned a lot and gained a lot of confidence on this bike. But I could have learned the same for far less dough. Eventually, though, my back began giving me trouble and I was convinced it was because of the fact that you can't use your legs for any support on a cruiser - it's all on your butt.
2002 Triumph Thunderbird
I had seen the new Bonnevilles rolling around and was turned-on to Triumph. I bagged a used Triumph Thunderbird sold locally, and this is the bike on which I learned the art of operating a motorcycle. Late turn-ins, leaning-off, executing in the twisties. This bike could handle it and I was taking trips with more experienced friends that put this bike in the right place at the right time. I did my first two track days on this heavy-weight limo of a road racer, and learned it was time for something that was meant to be thrown into corners.
2001 Suzuki SV650S
So I traded the Triumph for an SV650 in 2007. Completely unequal trade, but if you haven't noticed thus far, I don't spend smart money on motorcycles. This bike saw a bunch of track days and trips to the Smokies before I decided to replace it with a supersport.
2007 Suzuki Suzuki SV650
This is my wife's bike, but I have the luxury of using it quite often. She got it while I had my SV650S, and we've since outfitted it with hard bags. It's so approachable, civilized, powerful, and utilitarian. Easy to work on, and you can even food shop with it. Unbelievably refined, economy motorcycle.
2006 Kawasaki ZX6R
My current machine. It finally saw a track day in November 2010, and it taught me how at home you can be operating a motorcycle at the race track.