555 Darmanitan(Standard)

Incinerate is generally recommended for its considerably faster energy generation compared to Fire Fang, but the latter offers slightly more damage output. Tackle is useless. 

Rock Slide is an efficient charge move and provides valuable coverage. Overheat costs a moderate 55 energy and has the potential to deal massive damage if unshielded, but a -2 ATK self-debuff makes it risky to use. Psychic costs the same amount of energy, but its coverage is less useful in most formats. Focus Blast is often too expensive to use energy-wise, but its coverage and high damage could be significant if unshielded. 

Darmanitan is a mono-Fire type with high offensive potential, poor bulk, and a somewhat cumbersome moveset. Its high ATK stat, access to Fire Fang, and STAB on top of it all lets Darmanitan mow down opponents who are weak to Fire, without needing to use a charge move. Poor energy generation limits the use of its powerful charge moves regardless, while a lack of coverage against its Ground and Water weaknesses can result in hard losses against those typings. To add insult to injury, Water Pokemon are among the most common typings in this league, which doesn't help Darmanitan's viability. 

Darmanitan's typing and fragility discourages its use in Ultra League. While Fire typing would have advantages against popular Steel and Fairy Pokemon, Darmanitan's poor bulk and sluggish moveset can cause it to struggle in seemingly advantageous matchups. It also has no chance against Giratina. 

With a max CP of 3105, Darmanitan lacks the stats that many legendary and pseudo-legendary Pokemon possess in Master League. While its Fire-typing isn't too terrible, its fragile bulk is its major downfall, causing losses against the majority of meta Pokemon in this league. Better to use Darmanitan for raids rather than PvP.