I'd say 90s. Although I'm sure some OGs still have major 70s Florida nostalgia, it would be difficult to make a case for pre-1982, or even pre-1990, since the resort was still growing with practically no subtractions. Once EPCOT Center's open, you may as well wait to let Horizons, Morocco, Living Seas, Norway, and Studios open. There was a period from the late 80s into the mid-90s when valuable new additions were opening and very few major classics had been done away with.
Selective timeline with a focus on the 90s:
1971 - Magic Kingdom opens
1973 - Pirates opens
1975 - Space Mountain and People Mover open
1980 - Big Thunder opens; Mickey Mouse Revue closes
1982 - EPCOT Center opens
1983 - Horizons opens; Swan Boats closes
1984 - Morocco opens
1986 - The Living Seas and Captain EO open; Magic Journeys closes but soon reopens in MK
1988 - Norway opens
1989 - MGM Studios, Wonders of Life, Typhoon Lagoon, Pleasure Island open
1992 - Splash Mountain opens
1993 - Mission to Mars closes
1994 - 20,000 Leagues, Captain EO, Magic Journeys, and Davy Crockett Canoes close; Tower of Terror, Honey I Shrunk the Audience, Timekeeper open; Kitchen Kabaret becomes Food Rocks; Snow White is made un-scary
1995 - Alien Encounter and Blizzard Beach open; Symbiosis becomes Circle of Life
1996 - World of Motion closes; Universe of Energy gets Ellenized
January 1998 - Dreamflight/Take Flight closes
April 1998 - Animal Kingdom opens; Tiki Room gets Gottfriedized; Main Street Cinema turns into retail
September 1998 - Mr Toad's Wild Ride and SuperStar Television close
October 1998 - Imagination goes under the knife; Fantasmic opens
December 1998 - Test Track opens
January 1999 - Horizons closes
February 1999 - wand added to Spaceship Earth
April 1999 - Sounds Dangerous opens
July 1999 - Rock 'n' Roller Coaster opens
October 1999 - Tapestry of Nations, Millennium Village, and Illuminations: Reflections of Earth open
November 1999 - Skyway closes
November 2001 - River Country closes
2006 - Everest opens
2007 - Cranium Command closes; Nemo in Seas and Caballeros in Mexico; Timekeeper becomes MILF
2012 - New Fantasyland opens
2014 - Maelstrom closes
2017 - Pandora opens; Great Movie Ride closes
2019 - Galaxy's Edge opens; Reflections of Earth closes
2022 - Cosmic Rewind opens
You can make a strong case for early 90s (before any of the CelebritySnarkCOT changes), but you trade off Tower of Terror or even Splash if you go back too far. 1993 would be a very good answer. On the other hand I'm also fond of some of the Y2K-era stuff like RoE and RNRC. But on balance, I'd say summer 1998: basically sacrificing World of Motion/Test Track and a few smaller EPCOT losses in order to gain Animal Kingdom, but threading the needle before most of the truly disastrous changes hit.
Think of it: if you're there that year you could go to Magic Kingdom and do Splash Mountain, Mr. Toad, Alien Encounter, and the Skyway(!)...then parkhop to EPCOT and do Horizons, real Figment, Cranium Command, and Maelstrom...go to Studios for Tower of Terror and Great Movie Ride...or to Animal Kingdom for Kilimanjaro Safaris and Countdown to Extinction...plus 3 water parks to choose from.