For me, it would be early to mid-90's. My parents bought into DVC literally the day it opened. We would get in to the parks for free...that stopped in 2000. But Epcot seemed to me to be at its best then. MGM Studios was still new. I would get engaged there in 1996 and our honeymoon was there in 1997.
I have been a lot with my kids since my oldest was born in 2000. Those memories overshadow memories of what the parks used to be...but I still recognize there was a resurgence in Disney Parks earlier that they have not yet recaptured. I'm hopeful we're on the precipice of that now.
There are many different opinions regarding sponsorship. The basic concept, whose value has been argued, is that these partnerships enable WDC to present attractions that they themselves are either unable or unwilling to support financially at that point in time.
On one hand, the guest gets more "stuff." On the other, the imagineering objectives get altered by the marketing objectives of the sponsor.
I'm incredibly influenced by the fact that I was a child in the 90's but I feel that was the time. However I would have loved to visit in the first year. My folks tell me about it and it sounds amazing.
The 1990s were the golden years, at least from an attractions perspective. Splash Mountain and Tower of Terror were opening while classics like Horizons and Imagination still existed. EPCOT was FAR better and the opening of AK in 1998 capped off an incredible decade.