Let's say you're a fan of the animated Justice League. You've seen everything from "Secret Origins Part 1" to "Divided We Fall" (or, if you're stateside and don't have access to YTV's episodes, "Hunter's Moon," which aired after "Clash," inexplicably). Think you know it all?
Well, you don't. Justice League's been building for awhile now, and here's what I'd consider a pretty comprehensive list of necessary viewing: origins, backstories, team-ups, first appearances, etc. This definitely isn't the "minimum required viewing," since the Justice League series is self-explanatory enough that you can just dive right in; no, this is more like the extended family of JL. These are the main (and not-so-main) ingredients that went into the JL series. Part 1 in an ongoing series.
Batman: Mask of the Phantasm: The only DCU animated feature to get a theatrical release, MotP still stands head and shoulders above anything Tim Burton or Joel Schumacher had a hand in. More importantly, this tells the origin of Batman, one of the Big Seven (even if he is only a part-time member).
Key episodes from Batman: The Animated Series:
"On Leather Wings" - The first DCU animated episode. Not much to do with the League other than introducing Batman, but I think that makes it necessary enough.
"Christmas With the Joker" - Arguably the first Joker episode (airdates suggest that "Joker's Favor" was first), and he's naturally a big player in several JL episodes.
"Joker's Favor" - A much, much better episode, and it introduces Harley Quinn to the world. She plays a part in "Wild Cards."
"The Clock King" - Naturally introduces the titular character, who becomes a member of Task Force X.
"Feat of Clay Parts 1&2" - Besides being fantastic episodes, these introduce Clayface, future member of Gorilla Grodd's "Secret Society."
"Moon of the Wolf" - Introduces Professor Milo, who has a minor role in "The Doomsday Sanction."
"Zatanna" - Though she doesn't demonstrate any real magical prowess here, Zatanna goes on to wave a wand and klat sdrawkcab in JLU episodes like "This Little Piggy." It's also important to note that Zatanna's the first non-Bat-family DCU hero to debut in the animated universe.
"Showdown" - We learn a bit about Ra's al Ghul's past, including his experience with a time-traveling lawman by the name of Jonah Hex, who would go on to appear in "The Once and Future Thing Part 1: Weird Western Tales." Or, actually, had already appeared in that, since he's clearly older in "Showdown" than in "WWT." Sigh...time travel's a pain in the butt.
Next: Gotham Knights and Batman Beyond!