。
He thought it was best to stay quiet.
Literal
Be-silent-is [nominalizer-の] [subject-が] number-one good [quotative-と] thought.
一番よい ('the best,' literally 'number one good') is the standard way to form a superlative in Japanese — Japanese has no morphological superlative, so 一番 ('number one') simply prefaces the adjective. The whole judgement is then quoted by と and embedded under 思った ('thought'). The の that nominalizes 黙っている is marked with が because it's the subject of 'is best.'