Dialga: The legendary Pokemon of time, found in the Spear Pillar.
Palkia: The legendary Pokemon of space, found in the Spear Pillar.
Giratina: The legendary Pokemon of antimatter, found in the Turnback Cave.
Uxie: The legendary Pokemon of knowledge, found in Lake Acuity.
Mesprit: The legendary Pokemon of emotion, found in Lake Valor.
Azelf: The legendary Pokemon of willpower, found in Lake Verity.
Cresselia: The legendary Pokemon of dreams, obtainable after defeating the main game.
Heatran: The legendary Pokemon of fire, found in Stark Mountain.
Regigigas: The legendary Pokemon of power, obtainable after defeating the main game.
Regice: The legendary Pokemon of ice, found in Snowpoint Temple.
Regirock: The legendary Pokemon of rock, found in the Valley Windworks.
Registeel: The legendary Pokemon of steel, found in the Iron Island.
Shaymin: The legendary Pokemon of flowers, obtainable after completing the post-game story.
Darkrai: The legendary Pokemon of nightmares, obtainable after completing the post-game story.
Manaphy: The legendary Pokemon of the sea, obtainable through an event.
Phione: The legendary Pokemon of the sea, obtainable through breeding a Manaphy.
While these Pokemon are considered legendary, there are also other rare and powerful Pokemon that could be considered legendary by some players. These include:
Rotom: An electric/ghost type Pokemon that can change forms by inhabiting different appliances.
Garchomp: A dragon/ground type Pokemon known for its speed and power.
Lucario: A fighting/steel type Pokemon known for its aura and strong attacks.
Ultimately, whether or not a Pokemon is considered legendary is a matter of opinion. However, the Pokemon listed above are all extremely rare and powerful, and are sure to be a challenge for any trainer to catch.