The swivel has always held well, but can create a problem when reeled into the guides. I have also had toothy fish, like bluefish, hit the swivel and cut the line just above the bite leader. The loop-to-loop always seems to hold, but is bulky, can slow your line down during the cast, and can create issues if reeled into the guides. Other knots, like the blood knot, don't seem to hold due to the slickness and small diameter of the braid.
The one method that has never failed me is the simple double uni knot. By using five wraps on the leader and seven on the braid, the two lines pull against themselves and never slip. This also creates a small, streamlined knot that will easily reel through most guides and cast without fail.
Here's a great Youtube video by "Zvejokliai" (who also created the cover knot image) that shows how to tie the double uni knot.