0078 Rapidash

Incinerate features superior damage output and energy generation on paper, but in practice its long animation can make it difficult to use. Fire Spin falls behind in energy gains; Ember is an even further downgrade. Low Kick sports the worst performance in both damage output and energy gains, and also lacks STAB. 

Drill Run is recommended for its wide coverage, efficient DPE (Damage Per Energy), and costs the least amount of energy among Rapidash's charge moves. Scorching Sands falls behind Drill Run with lower DPE and higher energy cost, though a gamble for an Attack drop might make it a decent gimmick with a bit of luck. Flame Charge demands less energy than other charge move options, and while it's not as powerful, it still provides a guaranteed +1 ATK self buff. Fire Blast and Heat Wave both cost too much energy to be worthwhile. 

Rapidash sports a mono-Fire typing with high ATK stats and consequently poor bulk. Access to Drill Run lets it stand out from other Fire-type Pokemon, providing valuable coverage, though somewhat redundant against Steel Pokemon. Incinerate is a double-edged sword, improving energy generation stat-wise on paper, but the delay of lasting 5 turns (the longest of any fast move at the time of its introduction) can result in inconsistencies when charge moves or Pokemon switches occur. Losses against Pokemon with high damage output (such as Dragon Breath users) are largely due to Rapidash's fragility, while ubiquitous Water Pokemon will always be an issue. 

Rapidash has a max CP of 2461, which just barely reaches the CP cap of Ultra League. Unfortunately the lack of bulk stats remain the primary reason for various losses, and while Fire-typing is favorable against Grass, Bug, Steel, and Fairy Pokemon (many of which are common in this League), it does have its shortfalls against common Water-types and Pokemon with superior stats and/or faster movesets. 

Rapidash lacks stats for Master League.