Whirl Islands Lugia .

After obtaining the respective Bell and obtaining the Silver Wing, the player gets to encounter Lugia in the underground cavern located at the Whirl Islands.

In Pokémon SoulSilver, the player obtains the Tidal Bell and Silver Wing after clearing the Rocket Invasion at the Radio Tower,
and defeating the Kimono Girls in Ecruteak City. The Lugia will be encountered at Level 45.

In Pokémon HeartGold, the player obtains the Silver Wing from the Old Man in Pewter City.
The Lugia will be encountered at Level 70.