I've enjoyed all of them to varying degrees.
Superman: Doomsday - Adam Baldwin and James Marsters are very good in their voice roles as Superman and Lex Luthor respectively. Anne Heche, voicing Lois Lane, goes for a breathy rat-a-tat-tat style of delivery which was jarring at first, but which grew on me on repeat viewing. Her voice work in this is widely disliked, though.
Out of all the DTV so far, Superman: Doomsday is the one that's closest to the design sensibilities of the DCAU. That's unsurprising since Bruce Timm is one of the directors and did a lot of the design work. The lines on Superman's cheeks are often cited as an eyesore in terms of the designs, and I agree that they don't look great, but it's not a deal-breaker for me.
One of the main sources of complaint is that the story is very different from the comic book storyline. Accepting that it's loosely inspired by the comics rather than anything approaching a direct adaptation is crucial to enjoyment of it.
Anyway, in my opinion the action is pretty good and the story is solid, while the highlights are to be found in the most perverse screen incarnations of Lex Luthor and Toyman seen to date.
Justice League: The New Frontier - the graphic novel is a big, sprawling work and much is lost by cutting it down to fit the running time of an animated DTV movie.
The negatives are that the story is choppy and may be hard to follow at times for those who haven't read the GN, some of the voice work is a little lackluster by the usually high standards of Warner Animation (although not overly so), and some of the big battle scenes lapse into poor animation in a clear case of biting off more than the budget can handle. The positives are mostly to be found in the great design work and the cool neo-Silver Age ambiance that's created.
Batman: Gotham Knight - a lot of people were disappointed that the various segments didn't fit together into a more cohesive narrative. Personally I thoroughly enjoyed it, but then I was perfectly ready to accept it as a series of vignettes that showcase Batman in different anime styles and are mostly thinly-connected mood pieces. Although he did a good job, it felt a little odd to me to hear Kevin Conroy's voice coming from these animation designs.
Wonder Woman - this is the one of the first four that I most highly recommend to buy. It has a strong story, good voice acting (with Nathan Fillion being the standout), great animation, some good humor, and a narrative that fits nicely within the running time. It also has a very rich color design that looks great in Blu-ray and a very good score.
