How to say "prime" in Japanese

• 最盛期さいせいきCommon
  nounCovers 'prime' as a peak period: the golden age, heyday, or height of prosperity of an era, nation, or enterprise — the time when its fortune or activity is at its fullest.
  その歌手かしゅはあの曲きょくで最盛期さいせいきにあった。↗

  The singer was at his best in that song.
• 最さいCommon
  taru-adjective, adverb (with と)Glossed 'prime; conspicuous,' but seldom the word you want: standalone 最 is virtually never used as a noun, being familiar instead as the superlative element meaning 'most.'
• 盛りさかりJLPT N3approx.Common
  noun, noun (suffix)The prime of one's life: a person's best days, when strength, youth, or beauty is at its fullest. Where 最盛期 names a peak period, 盛り pictures the person at their peak.
  彼かれは今いま男盛おとこざかりだ。↗

  He is now in the prime of manhood.

For 'prime' as a peak period — a golden age or heyday — use 最盛期; for the prime of a person's life, 「盛り」; the standalone 最 is marginal and almost never the word you want.

Usage comparisons are AI-assisted and cross-checked by multiple models; for study reference only

• 最盛期さいせいきgolden age
• 最たるさいたるprime
• 我が世の春わがよのはるheyday
• 絶頂期ぜっちょうきpeak period
• 最さいprime
• 春はるprime (of life)
• 盛りさかりprime (of one's life)
• 素そprime