In order of viewing
*CW's Arrow Universe
Timeline:
1. Arrow (Seasons 1 and 2)
2. The Flash (Season 1)
3. Arrow (Season 3)
4. Constantine
5. The Flash (Season 2 part. A)
6. Arrow (Season 4 part. A)
7. Vixen (Season 1)
-> Legends of Yesterday Crossover
8. Arrow (Season 4 part. B)
9. The Flash (Season 2 part. B)
10. Supergirl (Season 1)
11. DC’s Legends of Tomorrow (Season 1)
12. Supergirl (Season 2 part. A)
13. The Flash (Season 3 part. A)
14. Arrow (Season 5 part. A)
15. DC’s Legends of Tomorrow (Season 2 part. A)
16. Vixen (Season 2)
-> Invasion Crossover
17. Supergirl (Season 2 part. B)
18. The Flash (Season 3 part. B)
19. DC’s Legends of Tomorrow (Season 2 part. B)
20. Arrow (Season 5 part. B)
21. Supergirl (Season 3 part. A)
22. Arrow (Season 6 part. A)
23. The Flash (Season 4 part. A)
24. DC’s Legends of Tomorrow (Season 3 part. A)
-> Crisis on Earth-X Crossover
25. Freedom Fighters (Season 1)
26. Supergirl (Season 3 part. B)
27. DC’s Legends of Tomorrow (Season 3 part. B)
28. Arrow (Season 6 part. B)
29. The Flash (Season 4 part. B)
30. Black Lightning (Season 1)
31. Freedom Fighters (Season 2)
32. The Flash (Season 5 part. A)
33. Arrow (Season 7 part. A)
34. Supergirl (Season 4 part. A)
-> Elseworlds Crossover
35. Arrow (Season 7 part. B)
36. The Flash (Season 5 part. B)
37. Supergirl (Season 4 part. B)
38. DC’s Legends of Tomorrow (Season 4)
39. Black Lightning (Season 2)
40. Supergirl (Season 5 part. A)
41. Batwoman (Season 1 part. A)
42. The Flash (Season 6 part. A)
43. Arrow (Season 8 part. A)
44. Black Lightning (Season 3 part. A)
-> Crisis on Infinite Earths Crossover
45. Arrow (Season 8 part. B)
46. DC’s Legends of Tomorrow (Season 5)
47. Black Lightning (Season 3 part. B)
48. The Flash (Season 6 part. B)
49. Supergirl (Season 5 part. B)
50. Batwoman (Season 1 part. B)
51. Black Lightning (Season 4)
52. Supergirl (Season 6)
53. Batwoman (Seasons 2 and 3)
54. DC’s Legends of Tomorrow (Season 6)
55. The Flash (Season 7)
56. DC’s Legends of Tomorrow (Season 7)
57. The Flash (Seasons 8 and 9)