Crisis on 2 Earths was good one-off. If nothing else, I think most people at least love the opening theme and James Woods' Owlman.
JL: Doom is another fun one-off that feels like an extended episode from the old Justice League cartoon.
The Dark Knight Returns, Batman: Year One, and The Death of Superman and Reign of the Supermen were solid adaptations too.
Under the Red Hood got Jason Todd to become many people's favorite Robin.
Wonder Woman (2009) & GL: First Flight were good origin stories.
Superman vs the Elite is fun if only for it laughing in the faces of all the fags who say "dude, what if Superman was evil and edgy?" which is so cliche nowadays.
JL vs TT and TT: Judas Contract are alright if you want more Teen Titan's story.
SS: Hell to Pay is a solid Task Force X story with the type of humor the series is knowned for.
JL Dark: Apokolips War was a nice send-off to the DCAMU started in Flashpoint Paradox, but to fully appreciate it you would need to have seen the majority of movies that take place in that continuity, which is a mixed bag. I listed some of the standouts already. The rest range from okay, to unremarkable, to plain bad (the Damian trilogy to be more specific).
There are lots more, but this should be enough to get you started. Being the DCfag that I am, I would recommend most of their movie line up. It's one of the few things that has not been tainted by woke politics over the years. There is usually something good to find even in the bad movies, which just makes you appreciate the good movies even more.