The toughest is Dupont Bomb Cloth, followed by Ballistic Cloth, leaving Cordura and Cordura-Plus a very close third. Tweeds are also made of nylon, and while they look and feel heavier they are vulnerable to slashing with sharp objects. Lighter nylons are also strong and are often used for backpacks and duffels because of their flexibility and lightness. This is a reference to the thickness of the nylon threads in that fabric. The heavier the denier the more rugged the fabric woven from it.
Larger zippers tend to be sturdier.
And the oversized pulls are great for a firm grip, and for running a small padlock through before you check in your luggage. Some zippers are made of interlocking nylon coils. Many luggage makers have adopted the rubber wheels originally created for in-line skates. They are quiet and wear like iron. Bags with 4 wheels are somewhat more stable, but the 2 wheeled bags are easier to pull behind you and maneuver better through crowded airports.