Pignite's typing is Fire and Fighting. You are probably going to want a STAB move, even if it's going to be walled. Besides, you only have room for one attack and that one attack is going to be walled no matter what type it is. STAB is the way to go, and there are several to consider:
This is probably the best attacking option. It deals good amounts of damage, with recoil being shrugged off by Rest. If you are worried about being walled by Heatran, let me tell you this: Pignite is going to be killed by a Talonflame or Crawdaunt long before it gets to Heatran.
Pretty much a weaker Flare Blitz with no recoil.
Fighting STAB that works well if you already have several Curses set up to negate the Attack and Defense drops.
Much like Superpower, only with no Attack or Defense reduction. Curse already cuts Speed, so the drop is not as worrisome.
It's not STAB, and it misses more than Flare Blitz, but it can act as a surprise check to Talonflame or Mega Charizard Y on the switch.
Kill Fairies, although Pignite is pitifully slow and will most likely get outsped. Iron Head is weaker than Iron Tail, but has superior accuracy and vice versa.
Hits Flying- and Water-types. However, it will almost always fail to KO and then Pignite will be swamped with retaliating STAB Scalds and Brave Birds.
Covers the likes of Machamp, Mega Lopunny, etc., but it sucks for doing anything else.
Pairs well with Curse, but is otherwise a poor attack on Pignite.
Hit Fairies. See Iron Head/Tail
Stone Edge/Rock Slide
Decent attacks overall, Rock gives some good coverage. Stone Edge is essentially a Head Smash with less power, same accuracy, and no recoil.
See Thunder Punch.
Horrible on a non-Sub set, but you can play mind games with Gengar and such.
As for Nature, an Adamant one would be most beneficial for these moves. However, Pignite has an okay Special movepool with gems like Focus Blast, Grass Knot, and Heat Wave/Flamethrower. It is also one of the only Fire-types to learn Scald, other than Emboar.