He was truly an old-school special effects man. He did with his hands what today's people do with computers. What are his most famous films? Most people would name "Jason and the Argonauts" and "Clash of the Titans". (Don't you think that the recent remake of the latter fell flat in comparison with the original?) I would add "One Million Years B.C." to that list. But it's unfair to make choices. All of his films have their merits.
There's little else I can say about this great man. Let me just quote remarks made by a few directors.
George Lucas: "Ray has been a great inspiration to us all in special visual industry. The art of his earlier films, which most of us grew up on, inspired us so much. Without Ray Harryhausen, there would likely have been no Star Wars"
Peter Jackson: "The Lord of the Rings is my 'Ray Harryhausen movie'. Without his life-long love of wondrous images and storytelling it would never have been made – not by me at least".
James Cameron: "I think all of us who are practitioners in the arts of science fiction and fantasy movies now feel that we’re standing on the shoulders of a giant. If not for Ray’s contribution to the collective dreamscape, we wouldn’t be who we are".