Logitech Trackman Marble FX - There's nothing else like it!
I used this when Logitech made it years ago. It is one of the best trackball devices I've ever used. For some inexplicable reason, Logitech decided to discontinue these devices and I've never been able to find one like it since.
The design of the assembly makes it extremely comfortable to use, and the large marble reduces fatigue and provides excellent pointing accuracy. Also, the thumb and forefinger contact points help as well, unlike many of the thumb-only trackball devices.
If you find one of these, don't hesitate to pick it up. They only come used and you'll most likely need a USB to PS2 port adapter, but it's well worth it!
This trackball keeps the wrist straight, and the hand in a comfortable position. The 4 buttons are convenient, and the ball is very easy to operate and doesn't stick at all.