I think this drama will be his masterpiece for sure.
This is definitely going to be his greatest work ever.
この場合、次のような言い方ができますよ。
ーI think this drama will be his masterpiece for sure.
「このドラマは間違いなく彼の代表作になるでしょう」
masterpiece で「代表作」
ーThis is definitely going to be his greatest work ever.
「これは絶対に彼の最高の作品になるだろう」
one's greatest work で「最高の作品」
ご参考まで！
カナダ 
