This page is a list of trackballs. There's no universal agreement on which is best, but i list them roughly in order with better reviews to worse.
First, read: Trackball vs Mouse, and How to Choose a Trackball?
L-Trac Laser Trackball
Highest quality. Biggest in size. Huge ball, and spins! Smooth scroll wheel, also spins! Laser tracking, no need to clean. 3 standard buttons. No need for any software to work. Minor complaints is that the scroll-wheel and middle button are hard to reach, and isn't as ergonomic.
Elecom DEFT Trackball with Scroll Wheel
Logitech M570 Wireless Trackball
Logitech's M570 trackball is the most popular, simply because of the name. Whether you like it depends on whether you like thumb operated trackball.
See video reviews at Logitech M570 Wireless Trackball
Kensington Orbit Trackball with Scroll Ring
this one is well-loved. Medium sized ball, only 2 buttons, medium priced, but best thing is the scroll ring.
detail ＆ video at Kensington Orbit Trackball with Scroll Ring
Logitech Marble Trackball
The Logitech Trackman Marble is a classic. Loved by 3D software users (Computer Aided Design). Common complaint is that it doesn't have scroll wheel. Best thing is the ball here, which can spin for 1 second.
See video reviews at Logitech Trackman Marble Mouse Review
Kensington Slimblade Trackball
Best design. 4 buttons. Huge ball. But there are complaints about the software. No scroll wheel.
for video, see: Slimblade Trackball
Kensington Expert Mouse
Classic. Huge ball. 4 buttons. Best thing is scroll-ring. But this is large, and tall. Ball got big friction, can't spin.
more photos at Kensington Expert Mouse
Elecom Wireless Trackball Mouse
This one is from Japan. Not much reviews about it.
Adesso Trackball Mouse
Kensington Orbit Trackball
This is $25. From the looks, it seems to be a fantastic one. But bad reviews. No scroll wheel. 2 buttons only.
This one just remindes me of Star Trek. Some people on YouTube modified it so the ball lights up.
Sanwa Trackball Mouse MA-TB39
Kensington Orbit wireless trackball
Not so good. Small. The scroll touch ring doesn't work so well. But if you need a portable trackball, this fits the bill.
Detail ＆ video at Kensington Orbit wireless trackball
Best Trackball: Microsoft Trackball Explorer
The best trackball ever, ever, is the Microsoft Trackball Explorer. (discontinued) For detail, see: Microsoft Trackball Explorer.