Serperior main here.
The best Hidden Power for Serperior is Fire. With Fire, Serperior can get rid of Ferrothorn and Mega Scizor as well as Air Balloon, Levitating, or part Flying type Steels. You're walled by Flash Fire Heatran, but Heatran is worn down by Stealth Rock and Leech Seed as well as Spikes if you're using those. Ground is not good as you can't do anything of note to the Stealth Rock resistant, Leech Seed immune Ferrothorn. Rock isn't good either. With it you absolutely cannot touch Ferrothorn at all, and are complete bait for Magearna and Mega Scizor. With HP Fire, you can get rid of most common Serperior checks, and then switch to a Ground type for Heatran, or Ash-Greninja for Volcarona or the Mega Charizards. Cinderace is stopped by A-Greninja and Landorus-Therian. With that out of the way, what does HP Rock hit? Pretty much just Zapdos, who doesn't have very good defense compared to other Serperior checks, so it gets hit hard by Leaf Storm on the switch.