I've watched most of the movies, and the ones I'd rate them best to worst, like this:
The Dark Knight Returns Part 1 & 2 - 5/5
Justice League: The Flashpoint Paradox - 5/5
Under the Red Hood - 4.5/5
DC Showcase Original Shorts Collection - 4.5/5
Wonder Woman - 4/5
Superman/Batman: Apocalypse 4/5
Justice League: Doom - 3.5/5
Justice League: Crisis on Infinite Earths - 3/5
Justice League: The New Frontier - 3/5
Of the shorts, I'd rate them:
Superman/Shazam: The Return of Black Adam - 5/5
Jonah Hex - 5/5
Green Arrow - 4/5
The Spectre - 4/5
Catwoman - 2/5
The other DC films I've seen, but didn't like were:
Green Lantern: First Flight - I didn't feel strongly in any way towards or against it, so maybe I'll give it another chance, but I just found it a little dull.
Batman: Year One - Really, really boring. The comic's great, but unfortunately it translates to a pretty dull film. If they weren't as faithful as they were, it could of been a lot more interesting. The whole thing's practically just narration (like the comic), and it all gets pretty boring quick. Plus, the voice acting's quite terrible, even Bryan Cranston unfortunately.
Superman/Batman: Public Enemies - An interesting premise, but ultimately the relationship between Superman & Batman was explored very interestingly, and the villains were horrible, especially Luthor. Just a really paper thin movie, that should of just been a tv episode of Justice League, or something.
Batman: Gotham Knight - A dull collection of shorts, from what I remember. It's been a long time, so maybe I'll check it out again, but from my memory none of them were at all interesting.

Young Justice is really good.

Nothing compares to JLA:Unlimited in my opinion. I haven't quite liked any of the DC animations since then. Though Wonder Woman and Superman/Batman Apocalypse was pretty good. I'd start with them.

The only ones I didn't like were "Death of Superman" (which was terrible) and Superman/Batman Public Enemies (which has a few good fights, but animation is bad and it's really quite silly in a bad way throughout).
This Superman/Shazam is my fave, because you get 4 very well animated, concise, short stories. Each had me going "Wow!" a few times each, Bruce Timm produced them all and the director is a Batman Beyond/JLA veteran and it feels very much in line with those, only better animated and bigger budget.
JLA: Crisis on 2 Earths is also awesome, great fights throughout. Superman/Batman Apocalypse probably has the best character design alltogether and the single best fight (wont spoil it) but feels lackluster while your watching it (until that amazing fight!).
Batman: under the Red hood is very good, but kinda dragged at times. Gotham Knights was very good. I haven't seen Wonder Woman or Green Lantern but I hear both are pretty good.
"Nobody knows anybody, not that well" - Miller's Crossing