Like a majority of other people, I think the medium-length series tend to work the best in terms of character and plot development, among other things. There are plenty of movies and shorter series that I absolutely adore as well, but when I think of my favourite series, a majority of them tend to fall in that middle category (Evangelion, Cowboy Bebop, Trigun, Wolf's Rain, etc.) There are a number of "longer" series (i.e., in my mind, pushing the 50-episode mark and above) that are still well-done that don't fall into the typical Shonen Jump style, 300+ episode category. In those fields, I'd throw Monster, Kenshin, both FMA series, and the two seasons of the Ghost in the Shell anime. All of those shows demonstrate that you can keep a series going for a long time without having it fall victim to the mindless filler in so many longer shows.(Naruto, Bleach, etc.) Of course, there's nothing inherently wrong with those longer shows, and I've watched sizable amounts of plenty of them, but, in terms of quality, I think medium-length shows tend to work the best.