Hmm, great question, OP.
If you'd asked this question in 2008 through 2011, my answer would have been YSL.
However, there isn't a single piece from the brand that I covet.
I like almost every bag Celine currently offers, except for the All Soft, which might change once I play with it. If I could only have bags from one brand, I would add (in order of preference) a Phantom, a Trio, a Blade, an Edge, an Envelope, a mini Luggage, a Trapeze, a nano Luggage, a clutch pouch, some jewelry. Almost everything but the furry shoes....
Wonder what my answer will be 3-5 years from now.