Best way or quickest way ?
For that I would make a black/white mask and you can do that in different ways from a straight paint inthe layer mask to making via various routes, the threshold filter might be a starting point or via conversion to an indexed image (just for a mask) ...and this only took a couple of minutes
or there is the FG / select tool. Invariably needs a bit of touching up.
For that image, I think I would spend a bit more time and care, trace the outline with the path tool, then path to selection.
For that I would make a black/white mask and you can do that in different ways from a straight paint inthe layer mask to making via various routes, the threshold filter might be a starting point or via conversion to an indexed image (just for a mask) ...and this only took a couple of minutes
or there is the FG / select tool. Invariably needs a bit of touching up.
For that image, I think I would spend a bit more time and care, trace the outline with the path tool, then path to selection.