Currently, it's a tie between my (1) YSL sac de jour (my main work bag, so used quite often) and (2) LV Metis Hobo.
Oh, and while it's mostly faded from personal use by now, I would say my most used bag ever was this charming tote bag from Ann Taylor Loft (the Gallery tote, I believe, which cost ~$130), which I used almost every day as my schoolbag for 4 years straight during grad school (graduated a year ago). It was a surprisingly durable and roomy bag, especially considering it was faux leather. Honestly, one of my greatest bag purchases ever and I have a special place for it in my heart, and that's including my Chanels. ^_^