I voted for Northanger Abbey -- I like the acerbic humour, and the protagonist who is naive and makes stupid mistakes, but is still described sympathetically by the author. I actually struggled through "The Mysteries of Udolpho" (which Catherine and Eleanor are raving over) because of it. (I don't recommend that one -- it drags on and on and on, and there's no satisfactory answer to the mystery.)
Others are close -- Pride and Predudice is probably second, or maybe Persuasion.
The only one I haven't reread more than once or twice is Emma, it makes me cringe from secondhand embarrassment
