According to this Gadgetwise column in the New York Times, Microsoft's Hotmail offers the best spam protection in the business. SITI, or spam in the inbox, rates have been reduced to record lows, with the advancement of Hotmail's Smartscreen technology. Reportedly, Microsoft has been able to keep SITI rates down to below 3%.
Microsoft also states that user complaints relative to SITI, including phishing, junk mail and malware, are down by 40% from last year. Cascade Insights confirms this number with their own report, which interestingly puts Hotmail and Google's Gmail neck and neck in regards to spam filtering, though ahead of all other competitors.