Happiness is a Warm Blanket, Charlie Brown is one movie that remains rather faithful to Schulz's style, since the animation was filmed in vignette style, with every section of the video adapted from the original Peanuts strips of the 1960's, as well as a flashback to the first strip from 10/2/1950.
In his will, Charles Schulz requested that the Peanuts characters remain as authentic as possible and that no new comic strips based on them to be drawn. The latter part of his will helps prevent unauthorized fanfic stories which would deviate from Schulz's style, yet CN's Mad still managed to pull off the "iCharlie" parody....
Nevertheless, I'd think that Warm Blanket seems rather faithful to animating the Peanuts characters the way Charles Schulz would have wanted it to be....