Of the 15 I've owned, I've definitely found some favorites.
First, the bikes I wouldn't buy again:
SV650 - No. Fun bike but not enough top-end power and not very versatile
Honda 919 - No. Didn't like the suspension or transmission but the engine was a gem.
Triumph Bonneville - No. Too slow for an 800cc twin and the seat was mighty uncomfortable. Couldn't bear to ruin the looks with an aftermarket seat.
Honda XR500 - No. Too old and too single-purposed.
SV1000S - No. Not enough power or suspension to make up for the sport-bike ergos.
Triumph Daytona - No. Lots of fun, fantastic motor, but my back can't stand the riding position.
Buell Ulysses (x2) - No. I did try this twice and liked the versatility, plus it was a blast to ride. The engine was too temperamental, however, and there wasn't enough steering lock for a bike that was designed to have off-pavement capability.
V-Strom 1000 - No. It was good on the highway but felt too top-heavy for off-road use. Had a bad stumble in the mid RPM range which had to be sorted with new fuel mapping. If I want a good highway tour mount, I'll just get an FJR1300 next time.
More of these, please:
2nd gen KLR650 - Yes. First bike I've had that could really go in the dirt and I've found that to be so much more fun than street riding. Though, this does street riding well too.
1st gen FZ1 (x2) - Yes. Very versatile with comfort, power, and reliability.
2nd gen FZ1 (x2) - Yes. Not as comfortable as the 1st gen but I liked having FI and it looked better.
Aprilia Tuono - Yes. Really enjoyed the power, looks, and handling. Would definitely buy one of the new V4 models.