Either the Logitech MX510 or the Razer Viper. People either love the Viper or hate it - some say it's a lot more accurate than the MX510, others say it generally sucks, dunno how or why exactly. IIRC it has something to do with the Viper being more accurate at high sensitivities and more or less unusable at lower ones. The MX510 is unanimously considered excellent. One other factor is the MX510 has a bunch of extra buttons, while the Viper only has 2 + scroll wheel. Your best bet would be to try both if you have the option, otherwise I'd get the MX510 to be safe.