The best replacement for arrowroot is cornstarch.

The best alternatives to arrowroot are rated as following:
★★★★★  cornstarch
★★★★  tapioca
★★  potato starch
★★  xanthan gum
★★  kudzu
★  cream of tartar
★  tapioca starch
★  baking powder
★  kuzu
★  cornflour
★  gelatin
  guar gum
  tapioca flour
  corn flour
  corn starch
  taro root
  agar
  pectin

Your alternative for arrowroot is not listed? Suggest!
Speaking about arrowroot, one probably shouldn't forget about cornstarch.

Other search terms related to arrowroot and their ratings are:
★★★★★ cornstarch
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Google Trends

Scale is based on the average worldwide traffic of arrowroot in all years. Click on graph for more.
arrowroot
cornstarch
tapioca