The right-wing noise machine wants people to think that McCain is some sort of maverick and that Obama is the most liberal Senator. However, VoteView objectively ranks Senators by an optimal classification method using all votes, and says otherwise. Here's how the Presidential candidates rank for the 110th (current) Senate.
110th Senate
10 (tie) Obama
20 (tie) Clinton
94 McCain
Similar rankings occurred in the 108th (2003-2005) and 109th Senates (2005-2007).
109th Senate
21 Obama
25 Clinton
100 McCain
108th Senate
21 (tie) Clinton
96 (tie) McCain
The 107th Senate was the last time that McCain didn't have an extreme right-wing rating.
107th Senate
22 Clinton
57 McCain