[ADDITIONAL COMMENTS]
<p>In terms of coverage, if using only one non-STAB attack Hidden Power Ice is probably the best choice, allowing you hit the numerous OU Dragons as well as Grass-types immune to Leech Seed. It still leaves Tangrowth walled by Fire-types, but their frequent use of Air Balloon and dual(not sure, I think there's a hyphen here too)typings means that Earthquake will often be worthless against them anyway, unless you can repeatedly predict their switches. Running both Hidden Power Ice and Earthquake, and dropping Leech Seed to allow for for better coverage but less healing can be viable, depending on your other Pokemons' needs. If your team can deal with Dragons without Tangrowth, Hidden Power Fire is a possibility over Ice to prevent Ferrothorn from coming in and setting up on this set easily. Focus Blast offers a way to hit Air Balloon Heatran along with being able to decimate Tyranitar, and if going physical or trying to avoid poor accuracy, Brick Break can take its place. However, Focus Blast fails to 2HKO standard Ferrothorn, Air Balloon Heatran can be dealt with by using Sleep Powder or by hitting it as it switches in, and Tyranitar takes a large hit from STAB Power Whip, making a Fighting move generally inferior to the other options.</p>