Mongo includes a mongoexport utility (see docs) which can dump a collection. This utility uses the native libmongoclient and is likely the fastest method.
mongoexport -d <database> -c <collection_name>
Also helpful:
-o: write the output to file, otherwise standard output is used (docs)
--jsonArray: generates a valid json document, instead of one json object per line (docs)
--pretty: outputs formatted json (docs)