Any early 80's big bore 2 stroke, with a huge rear knobby tire, bout as much fun as you can have with your pants on. I'm partial to the IT 465/490 from Yamaha.
VMax. When that V-Boost opens up, nuff said.
A big VTwin cruiser. Why not, damn near everyone else has. Gotta be a few good reasons.
KLR650. It's gotta be the cheap bastige Scottish in me.
Actually they are quite good for 2+ mile long wheelies at a constant 70 to 75 mph. They are cheap to own/maintain, and don't make you wince when they fall over. You don't have to wear the latest fashions, or buy the high dollar farkles, a milk crate will suffice.
Gotta say a scooter as well, less than 200cc preferably, especially for a second bike if you live in a place like Florida. They are a lot of fun, functional, a wonderful lesson in humility, and will make you think your other bike is a lot faster than it actually is...