Arthur and the Sword by Robert Sabuda. Atheneum, 1995.
With illustrations reminiscent of stained glass and medieval cathedrals, Sabuda distills the essence of the Arthur legend for young readers.
The Seeing Stone by Kevin Crossley-Holland. Scholastic, 2001.
There are a plethora of Arthur stories out there, but this is one of the best and most creative stories for older readers, Book One of his Arthur trilogy.