best friends/ the best friends

In the sentence: ‘We used to be/were best friends /the best friends before she moved to another town.’ which form is correct: best friends or the best friends?
Thank you for your time!

“we used to be / were best friends” implies that he/she was your closest / most intimate friend (and vice versa). “the best friends” is not very natural here, but you can say “the best of friends”, which just means that you were very good friends.