。
She's happiest when she's at home.
Literal
She [topic-は] home [at-に] is time [subject-が] number-one happy is.
A neat little sentence with a satisfying internal structure. The relative clause 家にいる ('be at home') modifies 時 ('time'); inside that clause, the subject is implicit (she). At the matrix level, 家にいる時が is the が-marked subject of 一番幸せだ ('is happiest'). 一番 ('number one, most') is the everyday spoken superlative marker, equivalent to the more formal 最も — 一番高い ('most expensive'), 一番好き ('like best'). The whole sentence has the warmth of self-knowledge, the kind of thing someone might tell a close friend.