Microsoft's Sculpt Ergonomic Mouse is one of those hidden gems that noone talks about.
The first thing you notice about this mouse is how tall it is. This is for a reason. The extended height allows your hand to be fully rested on the mouse and not have any portion touching your desk/mousepad. This is extremely comfortable. you can rest your arm's full weight on the mouse and you no longer have any friction on your hand while moving it.
The mouse is also tilted slightly to the right. This puts your hand on an almost vertical angle. Its tilted enough that you don't run into risk of carpal tunnel issues. But not fully vertical like some highly ergonomic mice, that you would need a full training period to use it.
Holding the mouse feels like gripping a ball that wraps perfectly around your palm. It is the most comfortable mouse I have ever used.
This is an ingeniously designed mouse designed by Microsoft Research. Due to its weird shape it doesn't seem to have gotten the popularity it deserves. However, I hope more manufacturers takes notice of this design and adopt it in higher end gaming mic.