naposletku

1. finally; in the end; eventually
   Dugo smo se raspravljali, ali smo se naposletku dogovorili. — We argued for a long time, but in the end we reached an agreement.
   Naposletku je ipak došao na žurku. — He did come to the party eventually.

It sounds slightly more formal than "najzad" and "konačno". For 'after all' introducing an argument, Serbian usually uses "uostalom" or "na kraju krajeva"; "naposletku" is also possible.