After looking him up, I see Bernie Sanders might be a bit more liberal than Barney Frank. Frank is rather extreme, but he seems even more extreme than he really is because of his explosive temper. Bill O'Reilly wins debates against him using the same tactics that Al Franken uses against O'Reilly. To find the most conservative member of Congress, I gave consistently right-wing answers to the VoteMatch quiz and looked at who matched those views most closely. The most conservative person in Congress might be Wally Herger:

http://www.issues2002.org/CA/Wally_Herger.htm