Brent McIntyre
From Tolkien Gateway
Brent McIntyre played the Witch-king in The Lord of the Rings (film series) film trilogy, in the scenes when the character is hooded (as a "Black Rider"). He also portrayed the Witch-king in the "Wraith-world" during the Weathertop scene in The Fellowship of the Ring.
When the Witch-king battled Éowyn in The Return of the King, the role was played by Lawrence Makoare.
At 6'10" (2.08 meters), Brent McIntyre was the tallest actor in the films.