From Longman Dictionary of Contemporary Englishthe best/greatest thing since sliced breadthe best/greatest thing since sliced breadinformal used humorously to say that something new is very helpful, useful etc He reckons his new mobile phone is the best thing since sliced bread. → sliced bread
Examples from the Corpus
the best/greatest thing since sliced bread• Now, I didn't get it because I was the greatest thing since sliced bread. 