Generally a Master and Servant participates in a Holy Grail War with the promise of a wish to be granted. in some cases like with Tokiomi Tohsaka and Waver Velvet the wish granting of the Holy Grail isn't their goal (the former sought the Root which is the main function of the grail, the latter just wanted to prove himself by winning) while other's like Diarmuid Ua Duibhne had their wish granted by being in the war.
However to my understanding with the Fuyuki Holy Grail Wars (Fate/Stay Night), the Moon Cell's Holy Grail War (Fate/Extra) and Tokyo Holy Grail War (Fate/Prototype) there has never been a victor and in later iterations something would actually prevent the proper granting of a wish (Fuyuki = Angra Mainyu's corruption. Moon Cell = Twice telling Masters to make his wish or be destroyed. Tokyo = Questionable if wish granting was ever possible, the true purpose was to summon Beast)
However i'm not entirely sure about some of the other Holy Grail Wars like the Subcategory Holy Grail Wars (Fate/Apocrypha, Fate/Labyrinth), the false Snowfield Holy Grail War (Fate/Strange Fake) or any other Holy Grail War i have missed as to whether or not they could and/or ended up granting someone's wish.
So has there ever been a Holy Grail War where the victor got their wish?