I guess your third option really does just boil down to which particular gimmick you want to back up your Nasty Plot boosted special attack. If you want a wall counter, Super Fang. If you want to use Gmork as an opener, Sucker Punch. If you need him to counter someone's ace preemptively, Embargo. If you want him out to absolutely set everything on fire, Sunny Day.

Personally I would say that this choice very much comes down to what kind of stat growth you get as you level him up. If he ends up with decent special defense then go for Sunny Day or Super Fang but if not, just pile on the speed and go for Sucker Punch or Embargo. If his physical attack stat keeps up, then Sucker Punch. If not then you're probably looking at Fire Blast/Dark Pulse/Embargo/Nasty Plot.

Of course, this is just my perspective. Nothing stopping you from making an angry demon dog that shoots chlorophyll laser beams if you really want.


^{look if you really want that SunnyBeam bullshit you just wait until Ninetales gets Drought}