Which Audio Research gears are classics?

The above responses are right on, I have never owned an Audio Research product I didn't like but had some that really stood out in terms of sound quality. I think the ARC LS-1 represents an excellent value for its used price but is often overlooked because there are no Audiogon reviews or posts on it. I have compared the LS-1 to the LS2 & found the LS-1 to be more transparent, with very good dynamics.

I like the D-75A, got one from a friend and restored it with 99.99% original parts. Its 70WPC and all tube, baby! Generates a lot of heat but it has been reliable; weighs a ton. Have not heard much about this amp here, but its very very nice, great sounding.

LS-17 imho. I had ls 2, ls 3, ls 7, ref 40
Currently ref 10 and ls 17

Amongst the pre-amps, the SP-3, -6, -8, -10, and -14 are particular favorites (I have owned the 3 and 6b, as well as LS-1 and -2). In power amps it is the D79. I have owned a D51 and D75, and much prefer the Music Reference RM-9 MK.2, which is available for around the same price as those two, I believe. Better sound, more reliable, longer output tube life.