Truckers' hitch / gun tackle
I use a variant of the hitch shown above, which has never failed me, nor has it failed to un-knot when I untie it.
After making the first slip knot, I half-hitch a bight of the running end around the loop.
This prevents the loop from closing.
After pulling the load tight, I half-hitch the running end around itself, but I use a loop in the half-hitch to ease release.
I then take the running end and half-hitch it around the release loop, to keep that from coming undone. As there is no load on that last loop, it will not jam.
JohnS