There are a couple things to consider:
1. Offensive production
2. Offensive compatibility
3. Defense
4. Durability
Offensive production - I would look at that as how many points the PG can create for himself or others. I think Rondo, Rose, CP, Deron, and Nash are all very good. If I had to pick I'd say Nash is 1, then Deron and CP, then Rondo and Rose.
Offensive compatibility - I think Nash could play with any four other players and fit in. He can shoot, he can run, he can ball handle in the half court. Deron is my second best guy, he can play off ball enough with his long jumpers (not really 3's though), then CP, then Rose, then Rondo. Rondo's lack of a J is a severe weakness in his ability to play with other guys that require the ball (and why Ray Allen is a great on court guard for him to run with.) But really all of these guys are going to be at their best in an offense where the PG handles the ball - so don't put these guys into the dreadful triangle, ever... which is where PGs go to die.
Defense - Rondo is vastly is superior here IMO. Deron is second because he uses his size well. Not sure about CP or Rose, I think they are okay defenders, but not great. Nash is bad.
Durability - Rondo takes a lot of licks, but seems to get up, and can pretty much go 48 if he needs to. This is important - when your team runs through the PG and you take the PG out someone inferior comes in, although a good GM (by getting good players) and a good coach (by having some other plays system to run) can mitigate it, its still an important part of the player evaluation. Rondo gets you a 19 PER for 40+ minutes when needed. You can't run Nash out there that much (IMO). I think Rose, Deron, and CP3 all fall between these guys.
My end take:
1. Deron
2. CP3 (assume he's healthy)
3. Nash (just too good offensively to knock him out)
4. Rondo
5. Rose
Didn't consider: Billups - he's very good. You could add him to any team as the PG and he'd fit in, but I don't see him as the starting point for any team - he's kind of like the current version of Ray Allen for a PG. There are other guys that might be ahead of Rose too, I'm just ranking the three "obvious" contenders, our guy, and the guy the thread is about.
Westbrook I just don''t know enough about him, but I liked what I saw in the playoffs.