。
Tanaka is the biggest fool in the whole department.
Literal
Tanaka [topic-は] department-within greatest [genitive-の] fool [copula-だよ].
省内きっての ('the most/greatest in the ministry/department') uses the superlative suffix きっての (切っての), meaning 'the foremost / the most X in a given group.' 省内 (しょうない, 'within the ministry') specifies the scope. 馬鹿者 (ばかもの, 'fool / idiot') is a blunt, somewhat old-fashioned insult — stronger than just ばか because 者 adds 'person' weight. The sentence-final だよ is casual-assertive. The tone is gossipy or frustrated workplace speech.