Experience with knots that break (telltale curly cue at the end of the mono), has led me to a knot that test close to 100 per cent:
Non-slip loop. This a knot popularized by Lefty Kreh that tests close to 100 per cent of the unknotted line strength. It is used for loop-to-loop connections (tippet to leader) and for attachment of a fly to tippets with a loop for added fly action. It is not a difficult knot to tie, but I would suggest you get a copy of his book, Practical Fishing Knots. The number of turns taken with the tag end around the standing line is dictated by the diameter of the leader.
Roop and Juro are "spot on" in recommending the palomar knot for tippet to fly connection that does not require a loop connection. It comes close to 100 per cent efficiency when tied correctly.
Two requirements for consistent knot tying is lubricating the knot and steady pressure when snugging the knot tight.
Tight lines and strong knots,