Most powerful h-bomb in Pacific history? I don’t think they’re quite to 15 megatons (operation castle).
Tsar Bomba (Russian: Царь-бомба; "Tsar-bomb", or "King of bombs") was the Western nickname for the Soviet RDS-220 hydrogen bomb (code name Ivan or Vanya), the most powerful nuclear weapon ever detonated. Its test on 30 October 1961 remains the most powerful man-made explosion in history. It was also referred to as Kuzkina mat (Russian: Кузькина мать; "Kuzma's mother"), possibly referring to First secretary Nikita Khrushchev's promise to show the United States a Kuzkina mat (an idiom roughly translating to "We'll show you!") at a 1960 session of United Nations General Assembly.
The bomb had a yield of 50 megaton TNT (210 PJ). In theory, it had a maximum yield of 100 megatons if it were to have included a U-238 tamper, but because only one bomb was built, this was never demonstrated. The single bomb was detonated at the Sukhoy Nos cape of Severny Island, part of Novaya Zemlya.