「！」。
"Aaah!" is an interjection.
Literal
"Aaah!" [topic-は] interjection [copula-だ].
A meta-linguistic sentence: the quoted form 「ああ！」 is the subject, and 感嘆詞 ('exclamation, interjection') is the category being assigned to it. Note 感嘆詞 is the linguistic term for an interjection — literally 'exclamation-word.' Common synonyms include 感動詞 and 間投詞.