The best replacement for topic sentence is thesis statement.

The best alternatives to topic sentence are rated as following:
★★★★★  thesis statement
★★★  main idea
★★★  thesis
★  thesis sentence
  topic
  thesis statements

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Speaking about topic sentence, one probably shouldn't forget about paragraph.

Other search terms related to topic sentence and their ratings are:
★★★★★ paragraph
★★★ example
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topic sentence
thesis statement
main idea