Final Draft
It seems like everyone I know uses Final Draft, though I know a couple of seasoned pros who still use a typewriter, one very famous pro who stills uses a typewriter, and several who just use Word.
I just use a Word Macro, and I've found it really doesn't matter much what you use. Everyone I've had to send screenplays to either wanted a paper copy or a Word doc, so the method I use to write the screenplay just doesn't matter to them.
The truth is, writing a screenplay is all tab and return, and I can write one faster without Final Draft. I can write one faster on a typewriter than on anything else, but getting good ribbons is a pain in the neck these days.