For the Italians, I would name the Battle of Caporetto which became synonymous with a terrible defeat. Hemmingway wrote about it in A Farewell to Arms. It was also one of few real examples that showed that breakthrough was possible in a war that was otherwise (in the west at least) defined by static carnage.

The battle was also important to Germany as it showed that Stoßtruppen and infiltration tactics developed by Oskar von Hutier (often called Hutier tactics at the time) really worked. The lessons learned at Caporetto, by the Germans in particular, could arguably be said to have paved the way for German tactics in WW2.

On another note, reddits favourite German General, Erwin Rommel fought at Caporetto. Leading only five men he managed to capture 1500 Italian soldiers. Pretty impressive (though it should be mentioned that Italian morale at this point was such that many units surrerendered willingly).

For Canada it could be argued to be none other than the Battle for Vimy Ridge. One of the main reasons why this wsa so momumental for the Canadians was the fact that it did much more for Canada than just a victory in combat.

To start, it would be the first time that all the Canadian Units would be fighting together under a Canadian commander, a huge step for the still young nation. It really helped to solidy in the Canadian people's eyes that they were among the best of the best in the world.

The success at Vimy would continue beyond combat too, as the battle would begint to represent the young Canada as a nation and help them to build the Canadian conscious.

For the Germans, the Battle of Tannenberg in August 1914 is pretty significant. It elevated Ludendorff and Hindenburg to the status of national heroes, as well as securing their promotion to the head of the German armed forces.

Gallipoli is important to the Turks as well as Australia and NZ, as it's seen as a decisive victory by Mustafa Kemal's Turkish nationalist army.