I think the best ones probably rank this way (in descending order):
Exclusive P3a ( Almost certainly the absolute best TT Pioneer ever made. Japan-market only, massive and beautifully-veneered plinth, over-engineered "statement" turntable.)
Exclusive P10 ('Lightweight" version of the P3a, but still a complete 'beast' of a turntable!)
PL-L1 (Ultra-rare Japan-market only linear-tracking "statement" turntable. It was the prototype/predecessor to the more affordable PL-L1000)
PLC-590 (Sometimes written PL-C590, and part of the "Series 20" line-up. The only turntable Pioneer sold without a tonearm, so audiophiles could choose to mount their own (SME 3009 was recommended) although Pioneer also offered two possible tonearm choices for it.)
PL-630 (Similar to PL-C590 in many important respects, but fully automatic.)
PL-L1000 (Pioneer's more successful, if lighter-weight, successor to the PL-L1)
I've SEEN every one of these models, and own two of them (PL-L1 and PLC-590). There are pics and information on most (all?) of them on The Vintage Knob website.
Correct but with one addendum.
The P3a was to the Exclusive line what the GT 2000x was to the Gigantic and Tremendous Line for Yamaha.A refinement of their Top Turntable(in Yamaha's case the GT 2000).
The P3a was a refinement of the Exclusive P3.
Similar to Yamaha's GTs,in that the P3a,P3 and P10 were never sold in Nth America,and rarely outside of Japan.Though apparently Europe may have seen a few of each model released(less than a dozen of each,possibly)
Arkay has possibly heard all of them(the models he posted about)and the Yamaha GTs.
I,and two other AKers(Badger and Mark.B) own GT 2000s.Badge and myself are in agreement that they are the best turntable we've heard.Mark hasn't used his yet.
The P3 is probably on a par with the GT 2000.The P3a is probably on a par with the GT 2000x.
The Vintage Knob has posted a group test of the 'Ultra Turntables'(of the era,not all-inclusive) from Japanese magazine Stereo Sound.
http://www.thevintageknob.org/MISCELLANEOUS/SS1980TT/SS1980TT.html
This should not be taken as the be all and end all test of Japanese Direct Drive turntables(nor the best of Japanese turntables,Belt or Direct Drive)because too many of the 'heavy hitters' are not included.
This was probably the best turntable of that era:
http://www.thevintageknob.org/THEVAULT1/SZ1/SZ1.html
There was one(supposedly) for sale on Audiogon (12 months ago?).Sold for $19,000 US.(Second-Hand)