The 1992 centenary documentary. Watch it before it gets taken down! http://www.youtube.com/watch?v=HkmNHP58OhU
Especially valuable for the very extended sections of Christopher explaining his father's world and philosophy. The part from about 7:00 on Secondary Worlds explains I think why both Tolkiens have been very unenthusiastic about dramatizations- a Secondary World can only properly exist in the mind.