이야말로 after a consonant (건강이야말로) · 야말로 after a vowel (저야말로) · 저야말로 감사하죠
truly, precisely, (it is) none other than
Example
건강이야말로 가장 소중한 재산이에요.
Health is truly the most precious wealth.
Word parts
An extra-strong form of the emphatic particle (이)야: 'X, if anything, is the one'.
Good to know
저야말로 'It's me who should (thank you / apologize)' is a very common polite reply: A: 감사합니다. B: 저야말로 감사하죠.