First off, I skimmed through the thread & didn't see much attention paid to current geometry.
2nd, road feel of certain tube sets
these are two huge contributors to the perfect frame. Now, there is going to be a handful of salt in there which is, "preference". This said, the two factors meeting in perfection are where I would consider the "peak" to be. Mid 80s to early 90's would be my answer. Why? Later frames have resorted to a relaxed head tube angle. Earlier frames were still in the era of refining steel alloys & lug shapes. Finally the lugged frame disappears after the point of a supple steel tube. Tubing wall thickness & diameter changed requiring an even further refinement of alloy which, in my opinion, completely ruined the feel of the steel frame. again, preference but, these tube sets just don't feel the same. Almost like a cross between aluminum & carbon. & with the oversized tube sets that had lugs made for them, much the same. The standard sized Reynolds, Columbus tubing sets of the 80's era coupled with a tight headtube angle, handled like nothing else. The softer steel, felt solid & smooth. & it could be built up quite light & responsive. The qualities carbon fiber bikes still Try to emulate.
Some modern advances, such as 15lb bikes, high grades of titanium & the ability to place materials in odd shapes & stregths are to much the benefit of carbon fiber bikes. But they still have yet to Atain the ride quality of the highest of the steel era.