The first instinct would be to think that the worst Entente victory would come in 1919 or 1920, with even more bloodletting and economic damage.
However, there is an argument that the British at least would have been pretty screwed with an Entente victory in 1915 or 1916, after Turkey entered the war but before Russia withdrew from the war. Because the only way this would have happened would have been if the "Russian steamroller" had been an actual reality, and Russian armies rolled into Berlin and the Russians got Constantinople somehow as well. That would have put Tsarist Russia in the same position of the Soviet Union in 1945. It was after all a fairly nasty regime, just not as much as Stalin's. And if the victory came in 1916 it would have been a very unstable but still expansionist regime.
For the British and the French, if they can't win a short war scenario in 1914, their best prospects are in 1917 or early 1918, after Tsarist Russia implodes but before the Americans show up in force. But its hard to see how they can do this. In a sense, Britain and France were screwed once their elites decided they had to prioritize stopping Germany, since they had to increase the power of either American or Russia to accomplish this.