Kurosawa. Speilberg has made some of my favorite stuff, but Kurosawa made Seven Samurai. Plus, without Kurosawa there would be no Star Wars. (That's right, fanboys. I said it, and I'll say it again: George Lucas created a great movie by ripping off a master director's idea.)
Oohh... tough call. Couldn't that kind of be a false choice because one is (kinda sorta implied to be) the reincarnation of the other?
If I absolutely had to choose though, I'd say Martin. Matthias is legendary too, but without Martin, Matthias wouldn't have become a legend, and Redwall might never have existed.
The White Witch (from the movies) or Morgana Le Fey (from the Merlin TV show)?