Legendary Pokémon are some of the most coveted creatures in Pokémon GO. Not every Legendary Pokémon is created equal, though. Based on the strength of their attack, the diversity of their moveset, and their usefulness in both raids and PVP, these are the Top Five most useful Legendary Pokémon in Pokémon GO.
There are Water-type Pokémon with spammier moves, such as Swampert with Hydro Cannon, but Kyogre is at the top of the aquatic meta because of how hard it hits as well as the moves it can get. This gargantuan Pokémon generally has Water-type moves, but it can learn the Electric-type Thunder, which makes it a useful counter to other Water-types as well.
This Legendary Pokémon is one of the most powerful in the game and, as a Ground-type, it can take on Steel-type Pokémon like Metagross and Dialga, both of which can be very nasty adversaries for an unprepared trainer. Like Kyogre, Groudon also has a spicy moveset with a Dragon-type Fast Move and options such as the Grass-type Solar Beam or the Fire-type moves Fire Blast and Fire Punch as its Charged Moves. A Groudon with Earthquake and Fire Punch, for example, would be able to defend itself against Grass-type Pokémon that would normally cause it a problem.
Trainers who have played in the GO Battle League know how much of a problem Giratina is. In its Altered Forme, it rules the Ultra League due to its defense and fast-charging moves. Its Origin Forme haunts the Master League with high attack and a mixture of Dragon- and Ghost-type moves that allow it to trick and overpower its opponents. It's also one of the top meta raid attackers, making it all-around one of the most useful in the game, Legendary or otherwise.
By far the most useful Dragon-type attacker in Pokémon GO until Kyurem Black enters the game, Rayquaza boasts Dragon-type, Flying-type, and Rock-type moves that makes it a force to be reckoned with in raids. Ice-types can cause Rayquaza trouble, but the damage it'll do on the way down is tremendous.
This iconic Generation One Legendary Pokémon has one of the most diverse movesets in the game, starting with its overpowered Legacy Moves of the Psychic-type Psystrike and the Ghost-type Shadow Ball. In addition to these, Mewtwo has five other typings in its moveset, and is able to learn the moves Flamethrower, Ice Beam, Thunderbolt, Focus Blast, Psychic, and Hyperbeam. While it's best to give Mewtwo one of its Legacy Moves, unlocking a second Charged Attack and giving it a Fighting-type or Ice-type move turns from useful into overpowered. It is, and has been for some time, at the very top of the game's meta.