I think the OP was just being silly, because he was looking at "playoff success"... meaning: Rondo won a title. That's like people who joked early on (prior to Miami's success) that Darko had a more successful career than Bron, Wade, Melo, Bosh, because he won a title as a rookie.
But if you're asking the question seriously...
I mean how young is young and what do we consider point guards?
obviously #1 and #2 is Chris Paul and Deron Williams. Then there is a MASSIVE gap.
Gotta put Tony Parker up there somewhere.
Monta Ellis is up there if he's a point.
Some outside of Boston would argue that Jose Calderon, Mo Williams, Devin Harris and Raymond Felton are all better than Rondo. Maybe you'd get someone who would argue that TJ Ford and Kirk Hinrich are better also.
As far as "who is the best prospect", you'll even get some people who will say Mike Conley and Derrick Rose both have more potential than Rondo.
But... maybe Rondo cracks the top 4 depending on how you want to look at it.