Dynamax Darmanitan

For Dynamax Battles, Fast Move and Charged Move damage doesn't matter much. The real key to winning is Max Move usage, so a Pokemon's Fast Move really is the key to victory. Fast Moves that can be used more frequently build up Max Move energy faster, which makes a 0.5 second Fast Move the gold standard, especially when the goal is to use Max Guard and/or Max Spirit.

It's also worth noting that a Fast Move's type is what determines the type of offensive Max Move a Pokemon will use. The exception to this is Gigantamax Pokemon, which will always use the same G-Max Move regardless of the type of their Fast Move.