Well, there's 3 "stages" of DC's Live-action Multiverse for starters:
1.) The 1st "stage" - Smallville (Earth-167) basically sets the stage for the Multiverse concept in season 10 and the Season 11 comics, although it never brings up other live-action continuities and it basically wipes out a bunch of nothing-Earths of the higher echelon to a smaller number by the time the Arrowverse begins - until Crisis on Infinite Earths.
2.) The 2nd "stage" is Post-Crisis. More on that when I have the time to explain what order, chicken-and-the-egg, that provides. It encompasses most of what I listed above. Some like Batman '66 existed before Crisis, but again, more on that later. Others are newly created for this "stage" of the Multiverse.
3.) The Flash film essentially causes the 3rd and final iteration of the live-action multiverse - Post-Flashpoint. This is a largely vague and so far unexplored territory/period for DC right now. The DCEU, DCU, Arrowverse (Earth-Prime) and Superman & Lois's unknown Earth are 100% part of it. Other Earths that existed prior to Flashpoint (but post-Crisis) presumably still exist or merged with other Earths. The specifics aren't fully explained yet, that's all for Gunn's team to decide for the future.
I'll give the viewing order later. Just wanted to explain the background some.