Some are worth watching, some aren’t.
The ones within the DCUAOM universe (Justice League: The Flashpoint Paradox, JL: War, Son of Batman, JL: Throne of Atlantis, Batman vs Robin, Batman: Bad Blood, JL vs Teen Titans, JL Dark, Teen Titans: The Judas Contract, Suicide Squad: Hell to Pay, and Death of Superman) are all generally good although a couple of them (Throne, BvR, Judas Contract) are a bit forgettable.
Gods and Monsters got some flack for its the ‘evil JL’ concept, but it really wasn’t that. It’s actually very optimistic. The comics tie-ins and Gods and Monsters Chronicles aren’t important to following the movie, but really expand it.
The standalones are a bit over the place.
The Killing Joke attempted to fix one of the major criticisms of the comic (treating Barbara like a prop)…but actually made things worse. It’s not a bad adaptation, if you ignore the prelude with Babs, though.
Superman: Doomsday is terrible. Attempting to get the Death and Return of Superman into a single movie was Not A Good Choice, and it really plays hell with the characters personalities - Superman and Luthor, especially, and most damningly. (Also…THIS MOVIE, not Man of Steel is a good example of Superman not even trying to take a fight out of Metropolis.)
The others based on specific comics, if you liked the original comics story they’re based on, you’ll probably like the movies, and vice versa, though not all of them stick closely to the source (Gotham by Gaslight, in particular, has a huge twist changing the identity of one character completely, and it works very well).
Crisis on Two Earths was meant as a link between the Justice League and JLU cartoons, but was done as a standalone, instead. The origin can still be seen in certain aspects, but it was very successful in creating a standalone movie. I’m a bit uncomfortable with a couple character deaths (or, rather, the other characters reactions to them), but that’s a minor complaint.
Batman and Harley Quinn was flawed, but watchable… (There’s some discussion about whether it’s in the DCAU or not.)
The ‘Knight’ anthologies are both pretty good - I prefer Emerald Knights to Dark Knight (the latter being set in the Nolanverse), but they’re both enjoyable.
Assault on Arkham is worth watching, even if you haven’t played the Arkham Asylum games.
The Wonder Woman and Green Lantern movies are both quite good - GL: First Flight especially stands well in light of the live action movie (and I say this as someone who generally liked GL), while WW suffers a bit in comparison to its live action counterpart (but is still quite enjoyable).
For some reason, not considered part of the DCUAOM line, despite being animated original movies based on DC properties, but the two Batman '66 movies - Return of the Caped Crusaders and Batman vs Two Face - are both a lot of fun - the latter being Adam West’s last work (and featuring William Shatner as Harvey Dent).