Russell Westbrook is the only starting point guard that I think plays better defense than Rondo.
Raymond Felton is a close call though.
Out of backup guards -- Kyle Lowry and George Hill are right there too. Rodrigue Beaubois. Oh, I forgot Kirk Hinrich. Starting at SG these days. He's a very good defender.
Possibly -- Keyon Dooling and Shannon Brown.
Anyway, I think Rondo is clearly in the top tier of defenders at his position. And in terms of starting PGs, I think he's either the second or third best in the league (Westbrook is #1).