Longchamp Penelope Medium in dark blue and Mulberry reg Alexa in black and SHW. Although the Penelope would be against some stiff competition from Hill&Friends Happy Satchel...
This is a tough question. The few specific bags I know want are ones I can get for myself so I wouldn't want to waste the blank check on them.
So I'd have to go with bags I wouldn't ever spend the money on even if I had it laying around: a limited edition Hermes Birkin and a limited edition (likely exotic) Chanel Boy or medium classic flap.