In most situations, the double fisherman's bend is the best knot for tying two ropes together.
Strong. Easy to inspect.
The double fisherman's bend can be hard to learn how to tie.
When the rope needs to run smoothly down a cliff, as in the situation of pulling a rope from the bottom, the double fisherman's bend is a poor choice because the knot can get stuck on ledges and in cracks. When the consequences of getting the rope stuck on a pull are greater than the risk of falling if the knot comes undone, use the flat overhand bend.