When you're tired, taking a quick bath is the best thing.

Literal

Tired time [topic-は] one-bath bathe [nominalizer-の] [subject-が] best [copula-だ].

ひと風呂浴びる means 'to take a quick bath' — ひと~ adds a casual 'one quick' nuance. いちばん is written in hiragana here instead of kanji 一番.