My favorite has always been the tried and true Microsoft IntelliMouse Explorer 3.0
It's a classic, and sometimes hard to find, but it feels awesome on the hand, has excellent response, great scrolling, and convenient side buttons. I actually have 2 brand new ones sitting on my shelf just so I won't have to buy another type for a long time after the current one takes a crap on me. I've used the current one for about 3 years now, the IntelliMouse Explorer I had before that, lasted me probably 4 or 5 years. I tried several brands after that and just couldn't get the same feel and comfort I got from the Intellimouse Explorer so I searched eBay till I found a seller who had several brand new ones for sale and bought them. Oh, I've tried other mice over the last few years as well.. so for about 8 years or more now, nothing has ever compared to the Microsoft IntelliMouse Explorer 3.0 for me personally.