Most are using the Blood Knot or the Uni to Uni knot. There was a segment on North American Fisherman where they had "Know Wars." These two were the toughest knots for joining two lines. Check out this website for some help:
http://www.animatedknots.com/knotlist.p ... dknots.com
I'm a blood knot guy, but only becasue I haven't invested the time in learning the Uni to Uni. However, I hear it is much easier to tie, and that may be a reason I check it out.