Pink Moon. Easily.
Not that the other two are bad, but the orchestrations of them just don't don't quite allow the beauty of his guitar playing, singing and songwriting to come through as well as it does on Pink Moon.
The only problem with getting Pink Moon first is that you may be let down by the next two. In that case, I'd say buy "Bryter Layter" first, which is better than Five Leaves Left, then buy Pink Moon to get the sense of the true greatnes. Then get Five Leaves Left to finish the collection.
My opinion. However you do it, get them all, and get them all NOW.