My list of "timeless" shows that illustrate the history of DCI and never get old:
Madison 1975
BD 1976
SOA 1980
27th Lancers 1980
Bridgemen 1980
Cadets 1985
BD 1986
Cadets 1987
VK 1987
SCV 1987
Cadets 1987
Madison 1988
SCV 1989
Phantom 1989
Star 1991
VK 1992
Cavaliers 1995
Phantom 1996
Cadets 1998
SCV 1999
Cadets 2000
Boston 00
All Cavaliers 00-06
Boston 02
Phantom 03
Crown 05
Bluecoats 06
Phantom 06-08
BD 09
Cavaliers 10
Cadets 11
Crown 12
Bluecoats 14
BK 14
BD 14 (not my bag but I can't ignore its significance)
The entire top 6 in 2015
Bluecoats 2016
Boston 2017
SCV 2018
Bluecoats 2019
BK 2019
I don't think we will get another classic this year unless Boston somehow pulls off Gold.
Intentionally left off my list: Cadets 05. I saw this at my very first DCI live show and while the corps talent was amazing the show design is nothing short of bizarre and creepy. I didn't enjoy it then and I still don't get it now.