best vs greatest thing since sliced bread

best

noun
  • The person (or persons; or thing or things) that is (are) most excellent. 

  • The supreme effort one can make, or has made. 

  • One's best behavior. 

verb
  • Had best. 

  • To surpass in skill or achievement. 

  • To beat in a contest. 

adv
  • To the most advantage; with the most success, cause, profit, benefit, or propriety. 

adj
  • Most; largest. 

  • Most superior; most favorable. 

greatest thing since sliced bread

noun
  • Something amazing, outstanding or praiseworthy, especially a relatively recent invention likely to improve people's lives. 

How often have the words best and greatest thing since sliced bread occurred in a corpus of books? (source: Google Ngram Viewer )