I bought a set of Axiom Monsoon Oceanweave 45 panniers about 5 or 6 years ago, been through quite a few horrible storms, and never once had any remote dampness get inside. Having said that, you should never trust any pannier not even Ortlieb, which I don't think they're the best, there have been a few complaints about the PVC lamination peeling but usually that's after about 5 to 10 years of use. Anyway, I bought a pair of Axiom rain covers for the bags, not so much to keep water out as to prevent abrasion wear. I also line the bags with white trash compactor bags because those are the toughest plastic bags you can buy, and white so you can see easier inside, then I roll those plastic bags closed before closing the panniers. I also use Ziplock bags to make sure things I don't want to get wet don't and things I don't want to leak out and get all over stuff ruined doesn't.

What panniers do I think are the best? Not mine though they are doing great so far, but I got mine on a huge, discounted sale price so that was my enticement. I do believe that the Ortlieb is better than the Axioms. However, I believe the best pannier on the market for long lasting durability is the Carradice Super C, these have a track record of lasting 20 to 30 years with rewaxing. They are made from 1000D cotton duck which is extremely abrasion‑resistant, which means there is zero chance of the lamination peeling, the bags are field‑repairable with needle and thread. The hardware is simple, metal, and rebuildable. However, they are not as waterproof, but if you line the bag with a plastic compactor bag everything will be fine. Although if you treat the bag as soon as you get them with a couple of extra coats of wax and reapply it each season you would be surprised as to how much water it will repel; plus, you can buy generic rain covers for them, Carradice the last time I checked does not make rain covers...odd. The Carradice weighs a tad more than the Ortlieb's, a potential downfall if you are trying to reduce weight as much as possible. I have a friend who has a pair of those Carradice panniers, and he claims he's never had anything get wet inside, but again, never take a chance, he doesn't take chances either, in fact he's the one that told me to use compactor bags as a pannier liner, and he uses a generic rain cover.

Another really nice set of panniers is the Vaude Aqua Back Plus, probably last about as long as the Ortlieb but it's a tad lighter than the Ortlieb if you are counting grams, and less expensive if you're counting money. They also use recycled materials which a lot of people like because it feels more responsible. Vaude does make rain covers for their bags.

Those are the only 2 bag companies that make something better or close to the same as the Ortlieb.

If the prices for those 3 bags are too expensive, there is a bag that is shockingly good for the money, those are the RockBros Waterproof Bike Panniers, these cost less than $55! The claim is that these are about 90% as good as the Ortlieb's!! RockBros does not sell a rain cover so you would have to buy the generic ones. So, you get 90% of the bag that Ortlieb is at a cost that is 70% less unless you get it on sale then it could be as much as 80% less! That is a solid deal if you are not wanting to use it a lot, but even with daily use the reported longevity is 4 to 7 years, only about a third less life than an Ortlieb would have, so in the long haul even if you replace the RockBros, you will come out ahead. These RockBros are noticeably lighter than even the Vaude bags, so if you are counting grams these are the panniers to buy.

Whatever pannier you buy it is wise to buy a set of rain covers regardless of if their Ortlieb's or whatever brand for the reasons I already mentioned, as well as using compactor trash bags inside, and Ziplock bags for whatever stuff.