I'll be extremely straight,
None of the elemental fangs are good on the classic Hippowdon.
As per how the OU goes, Hippowdown really doesn't have the speed to take a sweep, nor does it have the best typing for a tank.
They are mostly used as a wall and weather starter in that respect, and carry:
Although if you were to take a more offensive initiative, you should go for Ice Fang, as Ice is one of the best offensive typings in the game so far, and can hit many Pokemon for super-effective damage that would otherwise completely wall it (Eg.- Gliscor, Dragonite etc).
It doesn't give the best coverage as per Hippowdon's typing weaknesses, but most of it's weaknesses are special attackers, and because of Hippowdon's low speed and special defense stats, it will be knocked out by them anyways.
Hope I helped!