If you mean Physical STAB, there aren't many good ones.
- Best fire STAB for Primal Groudon are generally special moves, like Fire Blast and Eruption.
- Fire Punch and Heat Crash are decent moves but generally a rock-type with Precipice Blades forms the infamous EdgeQuake coverage which is generally enough. Physical Fire STAB doesn't hit many things as much as Precipice Blades+Rock-type move does.
The Smogon analysis written here doesn't recommend a physical STAB either. Fire Blast and Eruption mixed Primal Groudon is generally better.
Fire Punch doesn't hit anything special as much as Precipice Blades+Stone Edge covers. Generally in this range are:
Out of these, only Zacian-Crowned and Necrozma-Dusk-Mane are covered up by Fire STAB. However Fire Blast generally does more damage.
- 252 Atk Groudon-Primal Precipice Blades vs. 252 HP / 252+ Def Prism Armor Necrozma-Dusk-Mane: 229-271 (57.5 - 68%) -- guaranteed 2HKO after Leftovers recovery
This is done by Fire Punch. Moving to Fire Blast;
212+ SpA Groudon-Primal Fire Blast vs. 252 HP / 0 SpD Prism Armor Necrozma-Dusk-Mane in Harsh Sunshine: 447-528 (112.3 - 132.6%) -- guaranteed OHKO
252 Atk Groudon-Primal Fire Punch vs. 0 HP / 4 Def Zacian-Crowned in Harsh Sunshine: 420-494 (129.2 - 152%) -- guaranteed OHKO
212+ SpA Groudon-Primal Fire Blast vs. 0 HP / 0 SpD Zacian-Crowned in Harsh Sunshine: 572-674 (176 - 207.3%) -- guaranteed OHKO
This case both are over 100% so don't care about it :]
As such, the best fire STAB is Fire Blast, or Eruption if you don't mind the inconsistency of lower BP as lower HP.
Hope this helps!