Griffith, Chaplin, Keaton & DeMille are all silent film giants, but for me, the real "pioneer" for cinema is the great iconoclast Stroheim.

If you haven't seen his work, then ye gods, get yourself to a serious video store. His films are deliciously decadent- so sinfully sinister that you can't help loving them. He only directed 10 films (he's mainly known as a supporting actor) and only 5 are available on video:

Blind Husbands
Foolish Wives
Greed (originally a 9 hour epic)
The Wedding March
Queen Kelly