>>1817200For fabric, almost anything is strong enough (even lightweight fabrics), but most are too floppy. Four options, from least to most stiff. You can buy bags made from of all of these.
Ripstop nylon is the cheap and tough. Almost always has a urethane coating on the backside. Anything <500d is too floppy, 1000d is very common and home sewing machines can usually handle it. Cotton duck (like Carhartt jackets) works fine, esp if you wax it (pain in the ass), but you have to bind the edges or it will unravel (synthetics you can just melt seal). Vinyl is 100% waterproof, but very heavy. You'd want the stuff outdoor signs are made from, not car seat material. Laminates like X-Pac are expensive, but the plastic film layer makes it most stiff and waterproof wherever you don't poke holes in it.
I recommend ripstop nylon or laminate.
A #8 coil zipper is burly, you can pretty well manhandle it. Doesn't matter if
it's urethane coated (YKK calls it 'uretek'), or non-coated w/ a rainflap. They
use different sliders, but perform similarly. Neither will keep water out in a
driving rain.
A rain cover does better, and they're similar to ones for hiking pack. Sort of a
shower cap, usually silnylon (lightweight, good waterproof coating) with elastic
around the rim. If you use one, keep it somewhere you can grab & install in <20
seconds (a pocket on the outside of the pannier, handlebar bag, etc).
Extra features I suggest: top handle (like on a backpack; makes moving, adjusting bags much easier), exterior mesh pocket (temp storage of wrappers/trash, a place to dry out gloves/jacket/etc).
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