In what sense can both Laertes and Hamlet be regarded as victors in Hamlet?
This is a really interesting question to consider, and the way to approach it, in my opinion, is to think about how, although both Laertes and Hamlet obviously die, in their deaths they achieve peace with themselves and revenge against their common enemy: Claudius.
Laertes seems to undergo a kind of epiphany before he dies, when he realises how he has been used and manipulated by Claudius. His confession of their plot and then the way that he is able to see Claudius being punished and killed for his transgressions enable him to die "victorious" in a sense, because he recognises who the real villain is and exchanges forgiveness with Hamlet:
Exchange forgiveness with me, noble Hamlet;
Mine and my father's death come not upon thee,
Nor thine on me.
Thus having made peace with Hamlet and with himself, Laertes could be argued to die victorious.
Hamlet, too, can be argued to be victorious by the kind of death he achieves. He dies having avenged his father, relinquishing his kingdom to Fortinbrass whom he identifies as his natural heir and also ensures that Horatio will not kill himself so he can stay to tell Hamlet's story. His last line, "The rest is silence," can finally indicate a victory over the conflict that dogs Hamlet throughout the play, when he can embrace death because he has done what the Ghost asked of him. The sense of peace that Hamlet has at the beginning of the final scene in his conversation with Horatio is now fulfilled, and he can do what he has always wanted to do: to die and leave this "harsh world," as he tells Horatio.