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:
Flare Blitz
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.
Fire Punch
Pretty much a weaker Flare Blitz with no recoil.
Superpower
Fighting STAB that works well if you already have several Curses set up to negate the Attack and Defense drops.
Hammer Arm
Much like Superpower, only with no Attack or Defense reduction. Curse already cuts Speed, so the drop is not as worrisome.
Head Smash
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.
Iron Head/Tail
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.
Thunder Punch
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.
Zen Headbutt
Covers the likes of Machamp, Mega Lopunny, etc., but it sucks for doing anything else.
Gyro Ball
Pairs well with Curse, but is otherwise a poor attack on Pignite.
Poison Jab
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.
Wild Charge
See Thunder Punch.
Sucker 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.
