Barack Obama towers as the most far left progressive person running for president. And this isn't the spin of some enthusiastic supporter (in fact I've been leaning a bit towards Edwards since Michael Moore praised him), this is the objective assessment of the prestigious and authoritative National Journal which examined the totality of each candidates actual votes in three broad areas: Social issues, economic issues, and foreign policy issues.  When life time scores were compiled by the New York Times political blog, amazingly Obama towered over even ultra progressive Kucinich.

Senator Barack Obama......................84.3
Representative Dennis Kucinich........79.4
Senator Christopher J. Dodd .............79.2
Senator Hiliary Rodham Clinton ......78.8
Senator Joseph R. Biden, Jr...............76.8

Source:  http://www.majorityrules.org/blog/2007/0 3/senator-barack-obama-more-liberal-than .html

If all that weren't progressive enough, when you factor in Obama's Illinois senate record, he becomes off the charts progressive:

Illinois Senate
Health care: In 2004, sponsored the Health Care Justice Act, which called for a study of a single-payer health-care system for the state.
Minimum wage: In 2004, voted in favor of raising the minimum wage in Illinois to $6.50 an hour, up from $5.15 an hour.
Stem cell research: In 2004, backed legislation endorsing embryonic stem cell research.
Gay rights: In 2003, co-sponsored legislation banning discrimination based on sexual orientation.
Abortion: In 2001, voted against a bill designed to protect what backers termed live babies born during abortion procedures. The bill did not become law.
Ethics: In 1998, co-sponsored a bipartisan-backed package of legislation that overhauled state ethics laws.
SOURCES: Illinois General Assembly; U.S. Senate

Edwards has the most progressive sounding message right now, but if you look at the totality of their records and examine them objectively, Obama towers as the most progressive candidate. Not only is he the most progressive but he was progressive on the most important issues such as the war. Obama and Kucinich were the only two who opposed the war BEFORE it began. He has also been progressive in his willingness to meet with foreign leaders without preconditions.  Being progressive on foreign policy issues are especially important because they involve matters of war and peace, and affect the entire world, not just Americans.
Edwards is running a great progressive message right now, but I suspect that would change if he surged in the polls and had to start thinking about the general election. Hence I look at the overall record, and don't just cherry pick from recent rhetoric.  You can prove anything about anyone by selective and biased interpretation of data.  It's much wiser to look at the aggregate data, as compiled by an independent authority like the National Jouranl.