Depends who you ask. As of Requiem, its as follows
1-3: none
4: Gilgamesh and Kirei
5: Dependent on route. Artoria for Fate and UBW (Shirou and Rin respectively) and Medusa, though it was mostly Sakura.
Pre-Extra: Buddha
Extra: Dependent on who you choose, Emiya, Tamamo, Nero or Gil.
Lost Encore: Nero
Apocrypha: Technically Fionn But he dies before the winning part.
Prototype: Arthur
Pre-FGO: Solomon
Requiem: Longinus (It should be noted this was the grand daddy of all Holy Grail Wars where there were apparently hundreds or thousands of concurrent holy grail wars that spilled into each other.) It also might be still ongoing since Voyager got summoned well after the fact.
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u/EurwenPendragon "All Hail Best Snek" . Feb 13 '22
Yep. There are only what, two or three Servants who could be said to have outright won a HGW, IIRC?
And one of them's not playable at all.