there are two major manufacturers of WATERPROOF bike touring panniers:
arkel and ortlieb.
arkel is excellent, unfortunately expensive and not easy to buy.
ortlieb gets excellent reviews and whenever you see a biker anythere in the world, the likelyhood is greater than 50% that ortlieb panniers are attached to the bike.
i have decided for the 50 liter cordura bikepacker plus in yellow:
those have external pockets and are easier to pack and access.
also the plus series have more flexibility to position the panniers and allow repairs without a tool.
ortlieb uses plastic hooks which can break. that is why every tourer should carry spare hooks