I use an Albright Knot
best knot for fly line to backing, i also use it in place of the blood knot or nail knot in every instance. never had one come apart or break
Another vote for the albright knot.
I like how simple it is to tie.
A few years ago I was out chasing around some steel on a day when the river was just starting to drop after a blow out. Second fish of the day hit and took off instantly under a large log that was running acorss the tailout. A second later the hook popped out and the fish was gone. Once I started to reel in my line I discovered that my line was caught on this log and wouldn't budge. Not wanting to loose my line (Rio Versitip), I waded out to the log into the rapidly moving chest deep water to see what could be done. As luck would have it, the line was wedged in such a way that while I couldn't pull it upstream towards my rod, however, I was able to pull the line downstream away from my rod. In the end I was forced to cut my line at the backing, pull my line from the log and re-tie the albright knot all while standing in nipple deep water that was roaring past me.