Justice Scalia is often regarded as the most conservative member of the court, since he helped found the conservative Federalist Society, worked at the relatively conservative law firm Jones Day, and taught at the conservative University of Chicago Law School. Justice Thomas, however, also has significant conservative roots, but more along the lines of natural law and theory promulgated by think tanks like the Claremont Institute.
Chief Justice Roberts is seen as a political conservative due to his service in the White House, but he has remained extremely silent on political issues generally and has occasionally endorsed more liberal causes, like assisting the homosexual rights advocates with legal advice in Romer v. Evans.
Justice Kennedy is perhaps politically more conservative than Justice O'Connor, but again, it's difficult to determine from their interpretations of the Constitution alone. Justice Kennedy seems to advocate greater social restrictions on procedures like abortion that Justice O'Connor would, but it's uncertain whether that's a political view or a constitutional view. Justice O'Connor was a Republican moderate during her legislative days.
Justice Souter is perhaps the next as we move toward the liberal side of the court. He was touted as a Republican, but he's quickly moved to support liberal stances on a number of issues, still expressing some caution and moderation.
Justice Breyer is next, liberal in his decisions and policies, but also extremely pragmatic, so much so that his opinions seems relatively conservative in the sense that they seek to preserve the status quo.
Justice Stevens and Justice Ginsburg are the two most ideological liberals. Justice Stevens has reflected that his political opinions have always conveniently aligned with what he believes the Constitutions says, suggesting that the liberal advocacy he writes really does come forth as political belief. Justice Ginsburg used to advocate for women's rights groups and on behalf of other progressive and liberal organizations, suggesting a strong ideologically liberal bent.