The best combo is probably a Tubus rack with Ortlieb small or large rolltop.
The virtues of this system include a strong, very lightweight rack with quality metalwork. No adjustable legs (does your frame change size?), Strong, stiff upper stays, rear lamp bracket. If you use mudguards, go for an open frame top for extra lashing points.
The bag is strong, waterproof and the mounting system is quick release, locking.
If you go downmarket you can still find these features.
Many riders use single panniers.
The virtues of this system include a strong, very lightweight rack with quality metalwork. No adjustable legs (does your frame change size?), Strong, stiff upper stays, rear lamp bracket. If you use mudguards, go for an open frame top for extra lashing points.
The bag is strong, waterproof and the mounting system is quick release, locking.
If you go downmarket you can still find these features.
Many riders use single panniers.