I use a backpack for short journeys and lighter loads and a single pannier for commuting loads over 3 miles. I use both panniers for heavy shopping trips.
My idea of a good pannier is large, single compartment with one external pocket. Tough material with good, light stiffening board on back and base, modern, locking, quick-release hooks, heel cutout profile rather than simple square cut.
The closure can be zipper, rolltop or flaptop. For commuting, a flaptop is best, they are easy to overload with bread or other big, lightweight items. You can stuff waterproofs into them very quickly.
Zippers almost always break and cant be overloaded. Rolltops are OK but hard to get into for a quick stop.