In order:

  1. The Flash Season 1 Episode 8 (Flash Vs. Arrow)
  2. Arrow Season 3 Episode 8 (The Brave and the Bold)
  3. Heroes Join Forces – The Flash Season 2 Episode 8 (Legends of Today); Arrow Season 2 Episode 8 (Legends of Tomorrow)
  4. Supergirl Season 1 Episode 18 (World’s Finest)
  5. Invasion! – The Flash Season 3 Episode 8; Arrow Season 5 Episode 8; DC’s Legends of Tomorrow Season 2 Episode 8
  6. The Flash Season 3 Episode 18 (Duet)
  7. Crisis on Earth-X – Supergirl Season 3 Episode 8; Arrow Season 6 Episode 8; The Flash Season 4 Episode 8; DC’s Legends of Tomorrow Season 3 Episode 8
  8. Elseworlds – The Flash Season 5 Episode 9; Arrow Season 7 Episode 9; Supergirl Season 4 Episode 9

Maybe not all of these crossovers are relevant to Crisis on Infinite Earths but they provide much needed background on the dynamics between shows and characters plus they are loads of fun but my official ranking of the crossovers is here: Arrowverse Crossovers ranked

In total we are talking about 10 hours and 30 minutes but if you find you are pressed for time then a quick watch of Heroes Join Forces, Invasion!, Crisis on Earth-X and Elseworlds is enough to give you the context you need.