I like boats built in the mid-70s. Old enough to be cheap, new enough to have some life left, built in an era before blisters and particle board joinery. Maybe it's just my nostalgia-fogged eye, but it seems like many of the designs of the 70s still look attractive, while many later designs just look dated a decade or so later.
Actually, the 1970's was the worst period for blisters as that was the time when the formulations changed in response to the OPEc induced global fuel shortages and was also a period when accelerators were still widely being abused, fabrics were mishandled, and resin/catalyst proporting were still pretty lax.
It was also the time when hull thickness lightened but before internal framing became the norm so the boats of that era flexed and fatigued more rapidly than any other period before or after.
It was also a time when boats still used non-marine plywood for interiors and finished it behind formica and false teak veneers dyed to look like teak.
It was also a time when deck and cabin hardware was very crude compared to today. (Remember winch farms, halyard jambers, pressure alcohol stoves.
Depending on the specific design, rigs had gotten lighter and more fragile, often depending on checkstays, babystays and running backstays to keep the rig up as well.
And it was also a period when the popular hull and rig designs made for physically difficult boats to sail. Amoungst those of us who owned and raced boats of the IOR era, the general low regard held toward boats of that era results from issues that go far beyond the Fastnet Disaster.
In a broad general sense, IOR era boats were designed to be sailed by large crews with their weight on the rail for ballast. They tend to be unforgiving and require both strength and skill to sail well. While modern sails and hardware can go a long way toward making them less physical to sail, they still are hard boats to push around and are more likely to rely on a skilled crew to save the boat rather than the boat saving the crew.
Back to the question of a period with a sweet spot age wise, With so many better designs out there, for similar prices, I would suggest that the late 1980's and early 1990's offered better built, better sailing, easier to handle and safer designs.