。
Nothing is more important than friendship.
Literal
Friendship [as much as-ほど] important thing [topic-は] not-exist.
ほど~ない is Japanese's standard way to phrase a superlative through negation — literally 'nothing reaches the extent of X,' i.e. 'X is the most.' Here, 友情ほど大切な物はない frames friendship as the unsurpassed thing, with the negative existence verb supplying the comparison's tail.