What was the best year for prog?

It's 1972 for me all the way, the reasons:

Yes - Close to the Edge

Jethro Tull - TAAB

Tangerine Dream - Zeit

Genesis - Foxtrot

Aphrodite's Child - 666

ELP - Trilogy
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Who the heck told you that? What is your source (re no good synths before mid 74)? Also, I think there's plenty of great sounding albums by the late 70s. I would say by the early 70s it got better (not late). Also, one of the best sounding albums ever made came out in the 60s. Can you guess which one?

of lesser interest for the topic of this thread.

I voted arbitrarily 1971, because it needed help.

Gnosis 2000 being temporaily out, I couldn't really check my cumulative ratings to see which year is actually better.

If looking at the number of Progressive Rock releases each year, according to RYM's statistics they were about:

                      1971  -  470

                      1972  -  530

                      1973  -  470

                      1974  -  400

                      1975  -  470

                      1976  -  450

( https://rateyourmusic.com/genre/progressive-rock/ )

The synths on Brain Salad Surgery (73), Relayer and Lamb (both 74) all sound fine to me and very advanced for when they were recorded. You could even go back a little earlier. 

However, I will say that I always found it unfortunate that prog started to die out around 77/78 because the synth sounds happening at time (which were still developing as you implied) would have fit right in with prog. You can hear it in a lot of new wave stuff. Blondie's "heart of glass" is a good example. Listen to the synth on that.