Hum, this is difficult, because they're both so perfect. Streetcar packs a greater emotional punch, but I love the writing in Cat on a Hot Tin Roof. I guess I would have to go with the latter, but ask me tomorrow and I might answer differently.
The Glass Menagerie is also great, and it's the play that got me into Williams. I've recently finished reading Night of the Iguana, which also impressed me a lot; it has some of the best writing he's ever produced, and some of his best characters too.