who makes the best type of fender covers for protecting my boat from being marked by my fenders?

Taylor Made Premium Fender Covers.

http://www.taylormadegroup.com/frames/fshome.html

Just a note of caution, those covers seem to pick up lots of little gritty bits along the way. Make sure you clean them regularly or they turn into little sanding sponges.

I tried the Taylor ones they where thin and the elastic stretched out . I bought the Ploy Forms and they are thicker ,softer, and have the draw string that want let it slide off your jimmy Double the price though.

I have noticed If your tied up in some ruff water and they rub the paint [not gel coat]you can still see where they rub the paint . Still better than the naked bumper against your paint job.