The best replacement for serenity is firefly.

The best alternatives to serenity are rated as following:
★★★★★  firefly
★★★  firefly first
★★  amoryn
★  millenium falcon
★  depends
★  michelin primacy mxv4
★  k2
★  breath of heaven
★  millennium falcon
★  turanza serenity plus
★  primacy mxv4
  tranquility
  poise
  turanza el42

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Speaking about serenity, one probably shouldn't forget about calamity.

Other search terms related to serenity and their ratings are:
★★★★★ calamity
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Google Trends

Scale is based on the average worldwide traffic of serenity in all years. Click on graph for more.
serenity
firefly
firefly first