I second that the Top Knot lines aren't great. The problem with polypropylene (and I saw someone learn this the hard way) is that diminution in strength isn't always visible, because of the UV resistance issue. Top Knot claims that they have mostly fixed this, but it's a matter of chemistry and the laws of physics so I'm skeptical.
As mentioned, Samson has really solid docklines -- I used them for years on my sailboat. I found that their nylon double braid wasn't the best at abrasion resistance, and so I used mostly three-strand nylon which they no longer make as pre-spliced docklines. Fisheries Supply (or Defender) will make them for you from spool stock.
From two old boating friends back East I recently discovered Miami Cordage. I'm in the process of getting some double braid dockline quotes from them; FWIW they recommended 1/2" for our boats. So far they have excellent customer service, a variety of colors, and reasonable prices (comparable to Samson or slightly cheaper). Haven't pulled the trigger yet but am likely to order lines from them in the coming weeks.