It would not do justice to acharon acharon chaviv to translate it merely as the Hebrew equivalent of “last but not least”, which, although alliterative, is rather flavourless. On the other hand, acharon acharon chaviv harks back to Jacob and his profound love for Rachel, a love that led him to cry upon first meeting her.
On his return to the Land of Israel, Jacob prepared to confront Esau. He lined up his family in order to present them to his brother. First the handmaids with their children; then Leah and her children; and finally Rachel and Joseph (Benjamin had not yet been born).
Leah’s party is described as acharonim, which in Hebrew usually means last. The Talmud therefore comments that the Torah also refers to the middle item as acharon. For it is an addition to whatever came first and acharon derives from the word acher, which means another. Rachel and Joseph are acharonim to Leah’s acharonim. They are doubly concealed behind two groups and are acharon acharon. Rashi tersely comments on this description, acharon acharon chaviv, meaning the last one behind two others is the most cherished.
Nowadays, the expression is used to mean “last but not least”, but with such a touching history, I much prefer it and use it even when speaking in English.