。
Nothing is as precious as friendship.
Literal
Friendship [as much as-ほど] precious thing [topic-は] not-exist.
貴重 ('precious, valuable, irreplaceable') is a な-adjective with a kango feel — 貴 (noble) + 重 (heavy, weighty). The ほど~ない pattern is Japanese's standard way to phrase a superlative through negation: '[X]ほど~ない' = 'nothing is as ~ as X.'