The best replacement for synonyms is antonyms.

The best alternatives to synonyms are rated as following:
★★★★★  antonyms
★★  antonyms dictionary
★  antonyms worksheet
★  homonyms
★  views
★  thesaurus
  views sql
  homophones

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Speaking about synonyms, one probably shouldn't forget about reason.

Other search terms related to synonyms and their ratings are:
★★★★★ reason
★★★★ hesitation
★★★ antonyms
★★ dissent
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