Masterpiece, Best Work
noun, な adjective
Something you make that oozes with greatness is your masterpiece. The very best work you'll ever make in your entire life.
けっさく
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
これはピカソの傑作だね。
This is Picasso’s masterpiece.