This is something I think about a lot, so my post is long.
Jared: A US Glaub is at the top of the heap, in my experience. Particularly if you're after a classic P sound. (Disclaimer: Among the boutique brands cited, I haven't played an AC or a Celinder.) I think their necks are the best I've played--Sadowsky is right up there in terms of feel, but I don't know how easy it is to get a traditional P tone out of them. And you can get a lightly used US Lakland Glaub for under $2k, easily. They seem to come up often enough, and some have J necks if you prefer that.
This Dark Starred Glaub I bought this past summer has the nicest neck I've ever played. Here's a bad picture.
I've played one Lull PJ, passive with Fralins, and it was a very fine bass. I've played several Valentis (one passive single-coil P, the rest were active Js), and Nino makes a heck of a bass. I don't put them quite at the same level as Laklands, but then again we're talking about a lower price point.
In the same ballpark are Fender's Fullerton-era reissues (1982-84, maybe 85). I have a '57 reissue, and it's a killer. Light and resonant with a comfortable 1.75" neck that's very flat. The only thing that can be a drag is the nitro finish on the back of the neck--feels a little sticky compared to an oil finish.
Beyond that, I agree with Johnny Alien's comments about Skylines and recent MIA Fenders. I have a couple Skylines, and they're pretty damn nice. In fact, here's a link to a Skyline Glaub converted to a PJ that I've personally played several times (one of the best Skylines I've played). Doug, the seller, is a friend and a great guy. He's also a bad photographer

, that CAR finish is really great.
http://www.talkbass.com/forum/showthread.php?t=583554&highlight=
I haven't tried any G&L P basses. Good luck with the search!