People who know me know I like a lot of things very passionately. One of these many things is the amazing DC animated univrse (DCAU) from the 1990s through early 2000s. From the minds of Bruce Timm, Paul Dini, and a few others, the DCAU is a stretch of several DC comics-based TV shows in the same continuity that are really, truly great. There are some pieces of fiction you love as a child that only hold up due to nostalgia when revisited as an adult. These aren't like that. These contain interconnected stories beginning with Batman, moving on to Superman, Static Shock, the totally original Batman Beyond and Zeta Project, and into Justice League/Justice League: Unlimited (which was actually used to tie up Batman Beyond's story after its premature cancellation).

The series move from early 90s era episodic stories to larger serialized stories, poke fun at comic book tropes, contain some of the most moving storytelling you'll see in any medium. Just the 14th episode, from 1992, Heart of Ice won an Emmy and established what we now think of as Mr. Freeze's origin story. It introduces what is still to this day one of the best examples of a sympathetic villain I can think of. The three-part Superman story The World's Finest is an excellent execution of the Superman-meets-Batman dynamic DC looks so poised to fail horrendously at with their upcoming Hollywood movie and later there is an episode which covers a character's death with such immense weight that I may never forget the line ""in the end the world didn't need a Superman just a brave one" (Apokolips...Now!, Part 2). If you're a fan of the character Harley Quinn, you owe that to the Batman: The Animated Series as well. Numerous other original and moving stories are told with the trappings of DC superheroes in the universe, making it all pretty easy to recommend.

Knowing you want to watch it, though, doesn't mean the universe is easy to navigate. A few years ago I wrote up a "watching order" to organize the several shows into a suggested viewing order for anyone intending to dig in. Instead of just storing the list on my own machine, it seemed reasonable to go ahead and put this up here. A friend asking to borrow Batman: The Animated Series, the origin of the whole DCAU, this weekend seemed like as good a reason as any to put this up.

DC Animated Universe Watching Order

Legend:

Film - This is an in-canon movie.

Optional - Clearly all of this is optional, but these are sections that aren't particulary important and more for completion than "essential" material viewing.

Batman: Mask of the Phantasm (Watch any time early during Season 1 of B. No real continuity issues, so you could watch before the season, but I think you should at least start with Episodes 1 and 2 of the show to get a feel for it before watching this movie.)
Batman: The Animated Series Seasons 1-2
Superman: The Animated Series Seasons 1-2
Batman & Mr Freeze: SubZero (Watch before episode 3 of Batman Season 3, Cold Comfort. Easiest to just watch before starting the season.)
The New Batman Adventures (aka Batman: TAS Season 3)
Batman: Mystery of the Batwoman (Movie. Actually made very late, in 2003 in the universe of New Batman Adventures.)
Superman: TAS (Season 3)
Batman Beyond (All 3 Seasons)
Batman Beyond: Return of the Joker (Movie. If you can, watch the "Unrated" uncensored version. If you don't have the option, the censored version is acceptable.)
The Zeta Project (2 Seasons. If interested, watch after Batman Beyond. No other tie-ins.)
Static Shock (4 Seasons. If interested, watch after Batman Beyond. Before Justice League.)
Justice League (2 Seasons)
Justice League Unlimited (3 Seasons) (This show should to be the last thing you watch in the series, so if you're interested in watching the optional series, watch those first.)