Worst and Best Vinyl Quality Era?

There's a very precise answer to your question: It depends.

There's no question that as time went on, the record companies cut more and more corners. The mid-70s saw the rise of audiophile labels like Mobile Fidelity, as well as direct-to-disk recordings. So that's a good indication that mass-market quality had fallen pretty far by that point. Things certainly didn't get any better in the 80s, as companies started diverting resources to digital (and cassette).

Except maybe for some boutique outfits, I don't think any labels were immune to this. I collect jazz records, and I certainly see the phenomenon even on a label like Blue Note.

Around 1973-74 at the time of the oil embargo. They stopped using virgin vinyl and started using recycled viny. Sometimes you could find pieces of the old labels in your records from the old records that were melted down.

They do that in Jamaica with old vinyls & vinyls that do not sell.

Check out some of the 200 gram/45 RPM stuff. Many audiophile type labels. SEE: http://www.musicdirect.com/c-506-vinyl.aspx

I started buying records when I was in high school; 1959. The early Mercury and RCA Living Stereo records were all good quality; I still have some and they are in good condition and sound great.

Some labels, like DG and London, always put out a quality product. Many cheap labels did not. This was always true. There were always economy labels that cut corners. In the 1980s, when CDs started coming out, a lot of really cheap re-issues of older records came out, and were poor quality.

An example of that is the Mercury "Golden Imports", which were cheap re-issues of the original Mercury Living Presence records of the 1950s and 1960s, and they were poor quality.

Most of what music direct sells vinyl wise is good quality, especially the MOFI label. Elusive Disc is another good site. With the vinyl revival a lot of good stuff is being reissued on 180-200 gram pressings, unfortunately at premium prices. A lot of new stuff is on 180 also, ie the new Norah Jones and Jack White platters.

If you like a lot of Jazz...he is a site with some very good blue note reissues in 45 rpm. 200g.

http://www.musicmattersjazz.com/index2.html

In the mid '70s is when RCA's vinyl quality dropped through the floor.
Their Dyna-flex crap, paper thin. The RCA vinyl from the '50s, '60s were much better. I have a few Mobile Fidelity platters and they are equally as thin as the Dyna-flex.

On the other hand the mixing during the '70s was generally much better than the '60s. I have some '60s patters that are the infamous "hole in the middle recordings"; or recordings that do provide 3-channel sound, but with all the instrumentation coming from only one channel (music~Left; lead singer~center; background singers~right). Not even close to providing a full soundstage. Since when did all the musicians stand inside a 3 ft circle?

Sheffield Labs had some very clean recordings.

Yeah. Sheffield had their "DirectDisc" offerings, which went directly from the performers to the vinyl disc. I have a half dozen or so of these gems, including one by Thelma Houston and Pressure Cooker. Great bass and great dynamics.

The story goes that sales of the then-new release by The Who "Quadrophenia" was hampered by the lack of oil for pressing copies of the album. This may have caught the attention of record companies for the need to use so-called "recycled" vinyl.
One of the first to go in this direction was R.C.A. and their "flex" records (so called because they were so thin that you could easily bend them).