For index finger operated trackballs, the CST series is nice (preferably the one with the "more button" ports). I didn't like the Slimblade, as I found the extra buttons in awkward positions.
For thumb operated, I've been using my MTO (Microsoft Trackball Optical) for over 8 years now. Huge fan. And all they've ever needed was a cleaning. Second, I would say the Logitech M570. It's much lighter, the ball is smaller, so it's less desirable. But it's wireless, has great battery life, and so far no issues with any of the buttons.