If you use Pokemon with a high attack stat, it's very good. Most of the punching moves affected by Iron Fist are fighting-type, so it would be obvious to use a fighting type Pokemon. Conkeldurr has Iron Fist as hidden ability and attack stat of 140, only 3 lower than Melmetal, who has the highest attack stat of The Pokémon who has Iron Fist as an ability.
Also, you could combine it with the item Punching Glove in Scarlet/Violet, which further increases punching moves damage by 10%.
There's no doubt Melmetal is the strongest user of Iron Fist, but I'd say you could get the most damage done with Conkeldurr. Conkeldurr with Iron Fist and Punching Glove boosts Focus Punch from 150 BP to 195 or 198 BP (depending on if Iron Fist and Punching Glove is multiplied together or separately). Furthermore STAB boosts damage by 25%. If it's super effective against the opponent, it can be increased even further by either 100% or 300% depending on opponents typing.
I can give you a concrete example of Iron Fists damage using the damage calculator from Bulbapedia: https://bulbapedia.bulbagarden.net/wiki/Damage#Generation_V_onward
(Based on a level 100 Conkeldurr with max attack IV, attacking a level 100 Alolan Raticate with max defense IV).
The calculation looks like this (I think): https://ibb.co/jg8Z2yT
Which equals to around 1300 damage.
Now, without Iron Fist as an ability (but still considering Punching Glove), the damage would be somewhere around 1100. Alas, not a huge difference, but we're talking extremes in this case. After all, a 20% damage boost is not bad at all.
