The 1938 version of _Tom Sawyer_ is pretty highly considered.  I rate no
_Huck Finn_ as palatable.

An odd one:  the Claymation film, _The Adventures of Mark Twain_, is, I
think, quite well done.  It gets a lot of things right (and some wrong).
The version of "Diary of Adam and Eve" adapts to the medium well, and "The
Mysterious Stranger" is frightening.  Also captures Twain's dark late days.
 I have actually shown this to students, who have also enjoyed it and found
it good.  Can be rented in the kiddie section at Blockbuster!