I like Carradice ones. Easy to fit and remove, lightweight, bomp-proof construction, and highly water-resistant. I taken 2 slides on my panniers , one along 10ft of road on a hill and they are still usable.
Modern panniers should have a quick-release system on the rack, and good stiffening on the back and base.
The opening system you like depend on your use. I need frequent access to my bag, so use a flap covering. Roll-tops and zippers take too long for me.