My primary dock lines are extra long, double braided soft lay nylon. They're as old as my boat (2001) and in great shape, because we don't tie up to docks frequently. Mostly to take on fuel/water/pumpout. Never long-term.

However I also carry a few extra lines that are just cut sections of old 5/8" 3-strand anchor line. These are used when we need to tie up to pilings or other rougher use, such as storm tie up.