Listed chronologically they are:
1979
1980
1983
1984
1985
1996
2006

Each of these teams achieved a number of common things including winning more than 10 games, winning a conference championship, having at least one really memorable marquee win, beating Utah, and not getting blown out.

FWIW, the following is my chronological list of the next best BYU teams that fell just a tad short of being great (for example, they suffered at least one ugly blowout loss, ended the season poorly, or lacked a really memorable marquee win, etc.):
1977
1981
1989
1990
1991
1994
2001
2007
2008
2009

Weak/weird Covid-induced schedule notwithstanding, the 2020 BYU team has all but cemented it's legacy on at least the second list IMO. And while the 2020 team does not have the opportunity to achieve some of the first list accomplishments, getting to 11 wins with a marquee win to end the season would put them there in my book.

As a side note, keep in mind that at least a few of the teams on each of the above lists played a schedule that was arguably not substantially more difficult than this year's schedule.