I use the Polamar Knot whenever I can~I believe (I may be wrong) it's been tested to have greatest resistance to breaking compared to other convententional knots.

http://www.steelheader.net/knots/polamar_knot.htm

When I can't wrap around the object I'm tying to I use the Trilene Knot that Sky Guy posted.
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"Yes, I would support raising taxes"--Kanektok Kid