Funny how you worded that. I feel more or less the same, although the division between masterpiece and the merely really great, for me, falls a little earlier. I thought "Daydream Nation" was great, but not quite up to previous levels. They held steady from there, mostly.
For me, these days, "Bad Moon Rising" and "Confusion is Sex" are the ones I go back to most, with "Evol" and "Sister" nearly there as well.
The very first album is really great too, if quite a bit different than what followed.
Did you ever get in to any of the side projects or off-brand releases? The Ciccone Youth things, the "Made in the U.S.A." soundtrack, the this or that other side thing whose names I can't remember? I don't mean solo albums per se, but records that were the band, but not really part of the proper Sonic Youth lineage.