[Crazy Jetty]

I've not seen all that many. But my favorite one... I think the best written one was Batman: Under the Red Hood for DC, and Doctor Strange for Marvel.
I've not seen the Superman/Shazam, or Superman Allstar, though.

Justice League: New Frontiers deserves epic props, though, and was my fav, until Red Hood, which I thought was just really well written, and genuinely emotional (Even though I wished it had ended with Tim Drake making a cameo)

Green Lantern: First Flight *would* have been the best one had they split the story up into two, or three movies. They tried to cram like, three stories into a movie that was slightly longer than an hour. Other than trying to do too much, I thought it was really good.