This will be provocative!!! And surely there are many you should TRY, (as decants) to get you started. And its truly all in the nose of the beholder, etc...
But, since I always learn the most from/ get the most from the "classics", I am going to say:
Y by Ysl
It's a masterpiece and a fine example of what a perfume and a chypre can do/ be. It's much easier to love that some other big classics, and I cannot imagine not owning it!
Eight years ago, I'd have said Rive Gauche, but I don't think the reformulation stands up to the honor.