USCAP is probably the most "research" oriented meeting.
It depends on your goals. (that does not mean much in the real world)
the quality of the meetings in terms of peer review and scrutiny (in terms of what is accepted) is
1) USCAP
2) ASCP
3) CAP
Totally agree. USCAP is far the largest and has the most residents going, but doesn't nec. mean it's the "best." Depends on what you're looking for. USCAP has more of an academic, cutting-edge sort of feel (but fortunately more translational research type...not hardcore, bench stuff). ASCP a bit less on this, but more resident-oriented (i.e. courses are more geared towards general, perhaps private practice pathologists). CAP's basically like ASCP but again less academic and more resident/general education oriented, but a bit more formal/fancy (at least in my opinion) than ASCP.
To sum up my assessment of the overall "feel" of the sessions based on the majority of dudes' outfits (obviously exceptions exist):
1) USCAP - tons of suits/ties
2) CAP - sports jackets
3) ASCP - slacks and a shirt
Best story I can give is sitting in a GI lecture at an ASCP course, roomed filled with obvious private practice folks and some residents, and the presented asked the crowd (as an intro to particular topic) "So how many of you have encountered a GIST in your practice?" In my mind as a resident at a larger academic center, I was under the assumption that a bunch of folks would be raising their hands...but saw only a few hands go up, in the room of around 100-200 or so. At USCAP I think things would have been different.
But having attended courses at both, I really enjoyed the more general review type courses at ASCP. I could very easily just show up a talk on say adrenal tumors and get a great, overall review, as if I'd just read up on say the WHO or AFIP. But at USCAP, I'd be a bit more nervous about picking a random section to attend. Their review is less detailed and often dives right into current issues/debates, topics you'd instead find in AJSP/etc. But it's great to solidify an area you're interested in, making you even more up2date.