When you are limiting yourself to just one bag, it is essential to save space. There are so many things now that roll, fold, and crumple into smaller sizes so there's no excuse! Most of them are also extremely light-weight which means you don't have to sacrifice anything else.
My personal favorites are the Longchamps Le Pliage bags which combine functionality with style, as well as the folding ballerinas. A folding umbrella and hanger will often come in handy.