Old faithful: I've had many mice over the years including 3 Logitech gaming mice — nothing of inferior quality — but the Kensington Expert trackball I bought back in 2009(?) is still my favourite to use. I've already replaced the ball bearings, and the scroll wheel needs some TLC, but it's still going strong. And remains the only device I've used to date that has never caused me wrist pain after extended periods.
(I have a CST trackball as well, but those buttons are horrifically mushy with uncertain clicks; shame as the trackball action is much smoother on that.)