I think Nash and Rondo are close, but Rondo's defense is so much better I can't put him above Rajon.
Rondo's defense is much better than Nash's, but Nash's offense is much better than Rondo's. Nash is a 50/40/90 guy who still averages more assists than Rondo. Nash scores more, gets more assists, and is much, much more efficient. To me, that makes up for whatever defensive advantage that Rondo has. Nash is getting old, but he's still an amazing point guard. People (not you) who think he's "washed up" are completely off base.
(Also, Rondo's defense is a bit overrated, I think. He still has trouble staying in front of his man, and gambles too much. He has the defensive tools to be a lock down defender, but he's only shown that in spurts. Derrick Rose schooled him in Game 1 of the Chicago series.)