I’ve used braided nylon, twisted nylon, twisted hemp, and twisted silk. I prefer twisted ropes for the marks they leave, just dead sexy. I prefer hemp over nylon for the bite (easier to hold knots), and generally use 30 foot lengths. But if you’re working in the rain, or otherwise getting it wet, nylon is the way to go.
I certainly suggest hardware rope to start with! It’s cheap and plentiful.
When you’re ready to upgrade to artisan made, hand-dyed rope… I may be biased here, but I think my rope from http://www.WitherAndDye.com is excellent.