The blood knot or nail knot work well for me.

The knots above are good and strong and do not create a nasty bulge in the reel. But unless you expect a fish to take up all your floating line on a run, it's not as important as what you use at the leader end of the floating line. Fishing in relatively small streams for most of the twentieth century, my grandparents never used backing at all. Smaller reels designed for smaller fish don't have the space for it.

I always put backing on my fly reels.I put the fly line on backwards(don't tie it),and tie the backing to it with a nail knot.Then I finish filling the spool with the backing.Then I pull everything out,and tie the backing to the spool with a surgeons knot.I've never gotten into the backing while fighting a fish,but I don't want to lose a big fish because I didn't have it.Also fly lines develop memory.With the backing the coils won't be as tight.