Dragonite is already famous for its higher stats in Pokemon Go, and many players bring Dragonite to challenge gyms. Also, it is one of the best defenders. Recommended fast move is Dragon Breath, and charge move is Dragon Claw.
Snorlax is a normal-type Pokemon but can use Zen Headbutt, Psychic-type move. It has higher power along the fast moves. Additionally, its charge move, Body Slam, has the third higher power in all the charge moves. Therefore, it is the best move for defending gyms.
Vaporen is used by many players for winning gym battles and also works very well as a defender. Even though its fast move, Water Gun, has its power decreased, it is still one of the most powerful fast moves.
Hydro Pump is a recommended charge move based on its higher power instead of Aqua Tail, which is usually used for gym challenges based on its DPS.