I'd highly disagree with the other answer, Flare Blitz all the way.
- 252 Atk Life Orb Sheer Force Darmanitan Flare Blitz vs. 248 HP / 240+ Def Nidoqueen: 278-329 (72.5 - 85.9%) -- guaranteed 2HKO after Black Sludge recovery
- 252 Atk Life Orb Sheer Force Darmanitan Fire Punch vs. 248 HP / 240+ Def Nidoqueen: 175-208 (45.6 - 54.3%) -- 5.1% chance to 2HKO after Black Sludge recovery
If you're going for a wall breaker, its clear Flare Blitz is the superior option. The difference in power is just too much to ignore (45 base power difference). The recoil isn't so troublesome due to Darmanitan already have crap defenses, whether it be at 100% of 50% HP (due to recoil), it ain't living anything. The idea of Darmanitan is to come in, f s up, leave. With Fire Punch, it comes in, leaves a small dent and then leaves assuming it doesn't feint because your attack didn't kill the opponent.
The recoil shouldn't be problematic because you shouldn't need to be spamming Flare Blitz non stop. Darmanitan has plenty of strong coverage moves for when a fire move isn't the best option. With Spin or Defog support, you'll be just dandy.
Flare Blitz is what makes Darmanitan such a hard Pokemon to play against, very little switches in on it with ease.