Hughes is #2 only for his length of dominance; he did not fight the kind of competition that exists at WW for Usman, or what was around for GSP. WW was fairly weak at that time.I think Usman has a good argument to be #2 personally. Hughes has some pretty soft title defenses and benefited from 170 being the lightest available option. Could go either way IMO.
I don't see Hughes doing well against well rounded fighters with similar wrestling ability and much better striking; there weren't any of those around at WW when Hughes was champ.