when … the most
彼は孫と一緒のときが一番楽しそうだ。
He seems happiest when he's with his grandchildren.
子供って泣いてるときが一番かわいいよね。
Kids are at their cutest when they're crying, aren't they?
走っているときが一番幸せだ。
I’m happiest when I’m running.
ギター弾いてるときが一番幸せかも。
I think I’m happiest when I’m playing guitar.