It all depends.
If you have bull rail wood docks, like are popular on the BC coast, then
@Ward H ’s suggestion of twisted 3 strand makes good sense.
I prefer nylon lines as
@Davidasailor26 suggests because they stretch when the wind is howling softening the yin & yang pull of the boat Vs dock battle. Get good fenders to keep the hull clear of the dock.
Length is key. You need to have lines long enough to tie to the provided cleats but short enough so if the line falls into the water it will not reach the prop when hanging from the bow.
I've used 5/8 braided nylon on my dock for years. Definitely strong enough to keep the boat secure through the occasional winter gales. Our slip has large cleats which makes it easy to tie onto. My deck cleats are much better with 1/2 braided which I am switching to this season. Mostly because WM had a big dock line sale and I got the lines and colors I wanted at half price. Keep your eyes open for such opportunities.
I‘ve purchased a lot of my boat lines from a supply company serving arborists and loggers. They always seem to have a bin of end cuts less than 100ft in a variety of diameters and materials. Perfect for the small and mid sized boater at discount prices.