In 2008, both Clinton and Obama spent a lot of time debating a single fateful vote she cast in 2002 in favor of George W. Bush's invasion of Iraq. But if you look at all the votes that were cast during the four years they served together in the Senate, it was Clinton who amassed the more liberal record.
In the 109th Senate, Obama was the 17th most liberal member (between Chuck Schumer and Tom Harkin), while Clinton was the 13th most liberal member.
In the 110th Senate, Obama was the 18th most liberal member (again one tick to the left of Schumer) while Clinton was again 13th most liberal.
Another way of looking at it is that of the two Democratic senators from New York, Clinton was the more liberal. Of the two Democratic senators from Illinois, Obama was the more conservative.
Of course, one could say that these kind of crude vote-agglomeration methods miss a lot of what matters. A single vote on Iraq was more consequential than dozens of votes on budget amendments. But this is the point. Clinton's reputation as a centrist Democrat comes largely from her foreign policy. On the economic issues that dominate congressional votes by volume, she's liberal.