〜きっての
the best / most … in (all of)
Noun (a group or place) + きっての + Noun
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きっての means "the top … in the whole of X": 彼は社内きっての実力者だ, "he's the most capable person in the whole company".
The first noun is a group or place: 社内 ("the company"), 学内 ("the school"), 町, 日本, 業界 ("the industry"). The second is a person or thing praised as the best: 秀才, 名店, 名門, 実力者.
It's written, admiring and a little old-fashioned, found in profiles and news. It's always positive.
Don't confuse it with ならではの, "unique to". A 京都ならではの店 is a shop you can only find in Kyoto, while a 京都きっての店 is the best shop in Kyoto.
Examples
彼は社内きっての実力者だ。
He's the most capable person in the whole company.
ここは町きっての名店だ。
This is the finest restaurant in town.
彼女は学内きっての秀才だ。
She's the brightest student in the whole school.
彼は日本きっての名門大学を卒業した。
He graduated from one of the most prestigious universities in Japan.
彼女は業界きっての切れ者として知られている。
She's known as the sharpest mind in the industry.