screen plays are movies. do you mean stage plays that are adapted into a motion picture? if so, then it really varies on the type of theater play the movie is based on.
if you mean best ORIGINAL screen plays, where the movie is created from scratch so to speak, then that is also up to the personal taste.
i enjoy some novels that were made into movies, both based on non-fiction and fictional work. Lone Survivor, American Sniper, Green Zone, Captain Phillips, and others.
but I enjoy some original screen play movies too. Inception, Bridge of Spies, Zero Dark Thirty, The Hurt Locker, Wall E, Up, Finding Nemo, and others.
It just really depends on what your taste is what you're looking for in a movie.
if it's based off a book that i'm interested in, i will always try to read that first and compare. like Harry Potter, LOTR, or 13 Hours for example.