Originally Posted by Jipe

+1: the rack designed to fit the bike is usually the best solution, universal racks usually do not fit optimally and are ugly.

I use rear pannier on mist of my bikes (not the Brompton, its a major drawback of this bike, its not possible to put any side pannier on its rear rack, there is not enough clearance for the heels while pedaling).

I like Vaude and Ortlieb rear pannier (I prefer Vaude excepted that Ortlieb has XL rear pannier that doesn't exit from Vaude). Both brands have excellent hooks that allow to mount and dismount the pannier instantly.