Not a viewing order thing, but what particular universes do you think warrant a reference and canonization?
Batman (1966-1968)
Shazam/Isis (1974-1976)
Wonder Woman (1975-1979)
Superman (1978-1987)
Swamp Thing (1982-1989)
Superboy (1988-1992)
Swamp Thing (1990-1993)
Lois & Clark (1993-1997)
Steel (1997)
Catwoman (2004)
Constantine (2005)
Nolanverse (2005-2012)
Watchmen (2009)
Jonah Hex (2010)
Green Lantern (2011)
DCEU (2013-Now)
Gotham/Pennyworth (2014-Now)
Krypton (2018-2019)
Titans/Doom Patrol (2018-Now)
Swamp Thing (2019)
Joker (2019)
For me, it's got to be:
Batman (1966-1968)
Robin
Wonder Woman (1975-1979)
Wonder Woman
Superman (1978-1987)
Supergirl
Lois & Clark (1993-1997)
Superman
Lois Lane
Steel (1997)
Steel
Nolanverse (2005-2012)
Non-actor
Green Lantern (2011)
Non-actor
Gotham/Pennyworth (2014-Now)
Alfred Pennyworth
James Gordon
Harvey Bullock
Riddler
Penguin
(No Batman because David Mazouz is not 29, let alone 50 based on Pennyworth's timeline)
Krypton (2018-2019)
Seyg-El
Adam Strange
Lobo
Titans/Doom Patrol (2018-Now)
Nightwing
Robin
Cyborg
Swamp Thing (2019)
Swamp Thing
Realistically, I can see Lois & Clark and Gotham joining, but not the rest. They failed to get Wonder Woman and Krypton in, unfortunately, although Deadline claimed that theres a chance without elaborating.