I've tried many, the logitech mx518, g9, g700, had a few apple mice, a few plain jane microsoft ones, the R.A.T., and a bunch of razer's.
Thus far, the one I keep coming back too is the death adder. Not because I'm a fanboy, I don't like razer, but it's the most comfortable mouse I have ever used, and I have never had an issue with it.