幸せ is the common everyday word for happiness. 幸 (さち) is less frequent in casual speech and carries a slightly more literary or auspicious nuance.
Etymology
The reading さち is of native Japanese origin, likely related to 幸い (さいわい). The kanji 幸 is used for its meaning of 'happiness' or 'good fortune'. The reading こう is the Sino-Japanese on'yomi, and さき is an obsolete kana usage.
