* Dialga: Dialga is a powerful Steel/Dragon-type Pokemon that has access to a variety of powerful moves, including Draco Meteor, Flash Cannon, and Roar of Time. Its high base HP and Defense stats also make it very durable.
* Palkia: Palkia is a powerful Water/Dragon-type Pokemon that has access to powerful moves like Hydro Pump, Spacial Rend, and Draco Meteor. Its high base Speed and Special Attack stats also make it a great sweeper.
* Giratina: Giratina is a powerful Ghost/Dragon-type Pokemon that has access to a variety of powerful moves, including Shadow Force, Dragon Claw, and Shadow Ball. Its unique typing also gives it an advantage against many of the Frontier Brains' Pokemon.
* Groudon: Groudon is a powerful Ground-type Pokemon that has access to powerful moves like Earthquake, Precipice Blades, and Solar Beam. Its high base Attack and Defense stats also make it very durable.
* Kyogre: Kyogre is a powerful Water-type Pokemon that has access to powerful moves like Hydro Pump, Origin Pulse, and Water Spout. Its high base HP and Special Attack stats also make it very durable.
* Rayquaza: Rayquaza is a powerful Dragon/Flying-type Pokemon that has access to a variety of powerful moves, including Dragon Ascent, Outrage, and Dragon Claw. Its high base Attack and Speed stats also make it a great sweeper.
Ultimately, the best legendary Pokemon to beat the Frontier Brains in Pokemon Platinum is the one that you are most comfortable with and that best suits your team composition and playstyle.