This one, >>Ortlieb Roller Plus. Lighter than the Roller classic but with the same durable construction. I've exceeded the manufacturer's weight limits many, many times with out a single issue. If your not put off by the price, this is the one you want.

This is what I had in my bag after a trip to the market yesterday, which is typical of the kind of weight it has to indure. Plus, a roll-up style bag can carry a lot more stuff in an emergency than a bag with a strap down, lid.

6x 19oz cans of soup
6pack beer
Large bag of frozen burritos...yum!
Large bag of frozen veggies
Ulock
Tool roll full of roadside essentials
30oz jar of mayo