Regigigas is potentially the ultimate Regi Pokémon, appearing in Brilliant Diamond and Brilliant Steel to let others know that there is a new king in town. While you can get Regirock, Reisteel, and Regice, Regigigas is the most powerful of the bunch.
This Normal-type Legendary has a large variety of moves, which means it can be quite adaptable depending on what you need in your group of Pokémon.
Regigigas is a post-game Pokémon, so you'll need to have finished the story, cleared the Elite Four, and have access to the national Pokedex. You will also need to have access to Ramanas Park on Route 221. Head into the park and then you will need to exchange Mysterious Shards for three Discovery Slate. You can find mysterious fragments in the Grand Underground. Take them to the Discovery Room and then you'll need to capture Regice, Registeel and Regirock.
Make sure all three are in your party, then leave Ramanas Park and head to Snowpoint City in the far north of the map. Go to the Snowpoint Temple behind the Pokémon Center and enter. Go down the stairs and when you reach the bottom you will encounter Regigigas. Regigigas is a tough fight but will be weak against the Fighting type, so try filling the available slots with strong options of that type.