Usually the liberal to conservative rankings touted by the media are from the National Journal.
FiveThirtyEight blog has a an article about a site that uses a statistically rigorous method for their ranking. The site is
Vote view run by UC San Diego Professor Kenneth Poole. The most liberal Senators according to this metric are Russ Feingold, Chris Dodd, Bernie Sanders, Sheldon Whitehose, and Ted Kennedy. Here's the rankings for Barack Obama, Hillary Clinton, and John McCain compiled by Nate at FiveThirtyEight from this site.
Congress Obama McCain Clinton
107th -- 57/102 22/102
108th -- 96.5/100 21.5/100
109th 21/101 100/101 25/101
110th 10.5/101 94/101 20/101
Barack and Hillary are pretty much average Democrats but
John McCain quickly shed his supposed maverick status after the 107th congress (he'd just lost to Bush during the 107th) and became one of the most conservative senators overall. Heck, Brownback and McConnell had more liberal records than
McCain. You hear the media touting Obama as the most liberal but you don't hear the touting of
McCain as one of the most conservative. We should start pointing this out to everyone.
Edited to clean up table