Figure 8 or Fishermans

The most common knot that I come across used as a stopper knot is the Figure Eight. It is fast to tie and does the job. The only time the Figure Eight doesn't serve its duty is when the line is much smaller than its fairlead hole. In times like this, a Figure Eight will pull through because it wasn't bulky enough. This is when the Fisherman's Knot reigns supreme!

The Double Fisherman's knot is very easy to tie, and can be untied after being loaded (with some effort). I personally like the Triple Fisherman's Knot over the Double Fisherman's Knot simply because of aesthetic reasons. It looks like a barrel on the line rather than a square. 

The other advantage of a Fisherman's Knot is they lay straight in the line. A Figure Eight will make a bend in the line after the knot has been loaded, which will make a kink in the line when flaking or coiling. The Fisherman's Knots simply lay quietly in line as they serve their purpose.

You can see in this comparison I tied, the Figure Eight looks sufficient from some angles but flat and insignificant in other views. The Fisherman's Knot always has considerable bulk and won't pull through a block or sheave. The Fisherman's Knot also lays flat along the line whereas the Figure Eight has a bit of a kink to it.