Mr. Martin and Mr. Quinn rate the current court (based on data up through late 2010) as the most conservative in their database based on the positioning of the median justice, the previous high having come in the early 1950s. Although Justice Kennedy is not extraordinarily conservative relative to all other justices who have served on the court, he is very conservative by the standards of the median justice, who has typically been more of a true moderate.
Silver’s got some nice charts, so go read his whole post. Among other points — the liberal justices on the current court are far less liberal than the leftmost flanks of previous courts. And one key caveat: These scores are highly dependent on what cases the court actually takes up.