It's very difficult even if we go genre wise actually. I will take my two favourite genres: eclectic and symphonic and stick only to relatively big names.
Symphonic
1970: Trespass, EL&P s/t
1971: Nursery Cryme, Tarkus, Fragile
1972: Foxtrot,. Close to The Edge
1973: SEBTP, BSS, Ashes Are Burning
1974: Lamb Lies Down on Broadway, Turn of the Cards
Eclectic
1970: H to He, Lizard
1971: Acquiring The Taste, Pawn Hearts
1972: Three Friends, Octopus
1973: LITA, In A Glass House
1974:TPATG, Red
Honestly can't choose one year each from either of these.
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Maybe 1972 for symphonic and 1973 for eclectic, but it's damn hard. If I did a similar exercise for tech-extreme prog metal, the results would be similar too.