I think for me it's a tie between my navy suede Proenza Schouler PS1 and, lately, my black Balenciaga Velo. I've been carrying these two bags the most. Even though the Bal was quite expensive, it definitely has a great cost-per-wear ratio.
I wonder if that is less common on newer kate spade bags. I've been happy with the quality of my more recent kate spade purchases with the exception of the one with a cloth strap. The strap got stained quickly. I learned not to buy a bag with a nonremovable cloth strap.I use my Kate Spade bags until the handles start to shred... which I now notice is common.