Do you need a disc-compatible rack to clear a chainstay-mpunted brake or a normal rack for a seat-stay-mounted brake.
Commuting involves a lot of on/off mounting so get a pannier with a good, reliable, quick-release-locking system such as Ortleib, Rixen and Kaul. Avoid hook and elastic, velcro and anything which requires to to handle the bottom hook. The hook in a QR-locking system should be one piece of plastic or metal with an additional locking system (if the lock fails, the hook remains fully functional).
Racks need to be compatible with your hooks. This should be obvious but not to rack-makers.
Good racks have rails made of one tube/bar with a round profile. The hook goes on top, the lock slips underneath. Some modern racks have sections of twin rod welded together. The hook goes on top but the lock is blocked by the lower rod.

If you fit fenders then use an open-frame racktop. You can lash over-sized loads easily. Solid-top racks act as poor fenders and make lashing more difficult.
Some racks feature a raised loop at the front to act as a stop for large loads. Tubus don't use this feature but Blackburn do. I can slide the rounded end of my shackle lock under the stop and use a bungie to provide quick, secure lock storage.

Some racks have adjustable legs. You never need to adjust legs on a rack so get a non-adjustable one that fits. Simple is best.