Quite a few under-rated animated films I like very much:
Dragon Hill 1 & 2 are both set in a world inhabited by dragons and connected to other dimensions through space and time. Very simplistic plot, but I love it most for how cheesy it is. Weird characters, rubbish villains, hilariously bad English dubbing... and oh god is it glorious, not to mention it's very nicely animated.
The sequel ups the cheese level significantly. There's a sub-plot revolving around road signs... in a fantasy world. Also, there's cake. Glorious.
On the other side of things is Dragon Hunters, a film and a cartoon series... both are good (both are also different stories), but I love the film for the way it fleshes out the characters and how unique the setting is... and oh man the animation is amazing. Some of the floating island sequences near the end blew my mind.
Most of all I love the way they make dragons unique (some aren't really dragons at all), such as the snot shooting slug, electric raptor dragons, the demon dragon made out of hundreds of bats... oh and Hector, who pees fire. Glorious.