There are three bight knots commonly used in climbing:

• overhand

• figure 8

• butterfly 

(Yep, I'm calling it a butterfly, and not an “alpine” butterfly. I don't see any need for the word “alpine”, and I like simplicity.)

If you're putting any significant load onto this knot, consider using the butterfly.

It's usually much easier to untie after being loaded than the other two, with the overhand usually being the most difficult. (Most big wall climbers, who regularly deal with heavily loaded knots, have known this trick for a long time.) In the photo below, we have a figure 8 on the pig, because that’s pretty much a permanent knot in your haul rope that stays there all the time.

Of course, “hard” or “easy” is subjective! Factors such as your finger strength, level of patience, whether your rope is old and crusty or new and slick, wet or dry, can all have an effect.

What about a bowline?

A bowline is also easy to untie after being loaded. However, it has a tendency to loosen if it's subject to repeated loading and unloading, and it also needs some sort of backup. Also, it's tricky to tie as a bight knot in the middle of the rope, as we're doing here.

So, in most recreational climbing applications, one of the three bight knots listed above is usually preferred.