The best replacement for em dash is en dash.

The best alternatives to em dash are rated as following:
★★★★★  en dash
★★★★  colon
★★★  comma
★★★  semicolon
★★  hyphen
★  ellipsis
★  parentheses
★  double dash
★  dash
★  an en dash
  double hyphen
  n dash
  em dash

Your alternative for em dash is not listed? Suggest!
Speaking about em dash, one probably shouldn't forget about or without spaces.

Other search terms related to em dash and their ratings are:
★★★★★ or without spaces
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Google Trends

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em dash
en dash
colon