//best compression settings for bzip2

bzip2 --help
   ...
   -1 .. -9            set block size to 100k .. 900k
   --fast              alias for -1
   --best              alias for -9

bzip2 -1 is the default value. The block size is the window size of the input to be passed to the BWT-Transformation. The bigger the window the better the compression, but the more time and space will be consumed. See also de.wikipedia.org/wiki/Burrows-Wheeler-Transformation.