A duffel bag doesn't need to be fancy. One zipper, one compartment, that's it. A water-shunning exterior helps if you set it down on skeevy sidewalks (not to mention locker rooms). Look for one that can moonlight as a gym bag, weekend carryall, and impromptu backpack.
WHY THIS ONE: The Patagonia Black Hole 60L swallows up more than appearances suggest (3,661 cubic inches). It has a padded bottom to protect breakables and an extra zipper pocket for small items. Take it boating or on a road trip: it repels water, has shoulder straps, and can be lashed down at the corners. (Also comes in 90- and 120-liter sizes.)