I've used a double fishermans knot, I'm not sure about the strength reducing qualities of this knot but it has held well for me so far.
http://www.animatedknots.com/doublefishermans/
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Copy and pasted from a pdf after a little googling.
Strong Knots
Blood Knot 85–90% (Barnes); 80% (Day)
Flemish Bend 81% (Frank and Smith)
Figure Eight Loop 80% (Frank and Smith)
Double Fisherman’s 79% (Frank and Smith)
Moderately Strong Knots
Butterfly 75% (Frank and Smith)
Bowline 60% (Day)
Overhand Knot 60-65% (Luebben )
A Weak Knot
Overhand Bend (My guess is less than 50%)
I may go and learn the blood knot....