MX518 gets my vote, had the MS Explorer v3 and it's not as good.
Logitechs customer support is excellent, when my 1st MX518 developed an intermittent fault after 18 months (surfing cursor) I sent it back to the place I bought it from (cough) who said they couldn't find anything wrong with it (I have to assume it was tested properly) and would have charged me £10 for the pleasure if they hadn't lost my credit card details. I phoned Logitech and explained the problem, the guy said "we trust our customers" and sent me out a new one asking only that I cut the cord on the old mouse and send them the USB connector as proof.