Probably Steven Spielberg. He's been involved in some way or another with at least 12 of my top 20 movies. Another one would definitely be John Williams. I can't believe the shear volume of scores he's done, amazing.
george's choice of Edith Head was great. I counted 48 movies I like that she worked on. (I'm sure there would be many more if I'd seen everything she worked on.) Anyway, I didn't want to reply until I found somebody who beat 48 titles. Well, I finally found the guy: makeup artist Robert J. Schiffer, who worked on 49 movies I like.