No-one, actually. But then nobody has ever won a war since 1865. Consider the following:
- Serbia: disappeared for a century. Not a winner.
- Russia: sensibly backed out halfway through and had a revolution instead. Didn't recover till 1990. Loser.
- Austria-Hungary: ceased to be an Empire, split into component parts, lost many of them, weakened to the extent that Austria took Nazi invasion lying down less than 20 years later. Not a winner.
- Germany: Lost its Emperor, became a Republic (so far so good), but was forced by Woodrow Wilson to pay for the war; did so in the only possible way by devaluing the currency; totally ruined by galloping inflation; all savings wiped out; Republic desperately unpopular; rise of Nazism inevitable. You haven't won when you've got Hitler.
- France: lost an entire generation of young men, the whole North-West of the country devastated. Got back the 2 provinces in the North-East that it started the war to get, but left the Germans so bitter that they came right back 20 years later. If that's winning, I'm a banana.
- UK: Permanently impoverished, lost world dominance and a vast empire over the next half-century, lost sovereignty of the seas, became America's poodle. Pathetic.
- Japan: similar to Germany, only believed it had actually won the war, which produced dreams of Imperial dominance which led to wars of aggression over the next 20 years and directly to Hiroshima. Ouch.
- Turkey: just plain lost. End of Empire, end of everything for three-quarters of a century.
- Poland: Ah. certainly gained a certain autonomy, but lost it again to the Germans and then the Russians next time round. Nice try guys, but no win this time.
- Czechoslovakia: Came into existence. Disappeared again 20 years later. Re-appeared as a Russian puppet. Vanished again in the 1990s. Where?
- Yugoslavia: Well, ditto.
- Italy: Permanently humiliated. Ripe for Fascism. Virtual civil war until next big defeat, political turmoil ever since. Hardly a win there.
- Greece: Pretty well smashed, as usual.
- USA: Best candidate for title of winner, but the cost of the war, once Germany defaulted on the payment, led to the Depression, and the precedent set by Wilson destroyed American isolationism, persuaded Americans that the world's business is their business, and led directly to the mess they are in today.