I guess it depends on what you mean by oldest...
The one I've owned the longest is my 07 anthracite city. I got her last may my bday and she was the bal that "popped my cherry" so to speak. I adore her and while I don't wear her constantly, I do still wear her and I think she's the one bag I don't think I could ever sell for sentimental reasons.
As far as oldest chronologically in my collection, I have recently gotten my HG 2003 dark caramel city and I am loving her to bits - I've worn her every day since i've gotten her!