last but not least vs lastly

last but not least

adv
  • An expression to start the last item of a list, emphasising that while it is listed last, it is just as important as the rest of the items. 

lastly

adv
  • Used to mark the beginning of the last in a list of items or propositions. 

  • Used to signal that the speaker is about to yield control of the conversation. 

How often have the words last but not least and lastly occurred in a corpus of books? (source: Google Ngram Viewer )