Arkel Tailrider: Love this thing. Holds my lock, tools, wallet, keys, etc.

Ortlieb Back Roller Plus: Kind of annoyed with these. Came to realize I don't like the 1 compartment design, and the waterproof membrane really likes to cling to things inside the bag, making it difficult to put in, or take out anything plasticy especially.
But they do hold a lot, and seriously do NOT move when riding: Hitting potholes, jumping curbs, not a rattle.
I like how easy they are to mount onto the rack as well, simple clip-on and off. Makes going and doing laundry crazy easy.

Still, want to replace them with more Arkel I think.