It would be my Proenza Schouler PS11 Mini in black. Such a perfect to-go bag when I don't have to carry around my books or my iPad and I just need to pack my phone, keys, sunnies and wallet.
Lately, my Hermes Garden Party tote in Gold -- simple, durable, beautiful (I think) but completely under the radar. Coming in second would be Hermes Evelyne -- fits less, but has the all-important cross-body strap. In the winter and other bad weather times, Longchamp totes get a lot of use.
I wish I could say my Hermes Kellys or Chanel flaps, but in reality, my life is super casual and I often need to carry more than those bags can comfortably hold.