For Slovenes I'll go with the Battle of Doberdob
Fought in August 1916 on "Soška fronta" (Isonzo front), it was one of the most bloody battles in ww1. Austro-Hungarian forces – mostly Slovenes and Hungarians – repeled Italian attacks on higher ground positions. The battle, part of the 6th Battle of the Isonzo, was fought on the Kras plateu, north of Tržič (Monfalcone). Italian infantry attacked from the lowlands and was always 10x or even 20x the power of defending troops. The 87th Infantry Regiment of Celje fought for 12 days on top of the plateu. The loses of its 10th battalion were around 1000 at the end of the fights. Italians, who tried to gain control over Triest and Gorica, failed several times to break the line although having greater manpower and superiority in ammonition and canons. In the end they, however, the Italians managed to seize the plateu, but for the cost of heavy casualties and they didn't force their way to Triest or Gorica.
The Slovene 87th regiment of Celje had a important role in defending Doberdob. The 10th battalion left only 76 men of over 1000 killed and wounded. Historians Veith and Weber state the 87th managing successfully the hardest fight, which was ever faught by an Austrian unit. Also, the men of the 87th were known for having the sharpest handweapons. At Doberdob they sharpened their knifes, bayonets, axes and shovels even during the battle, which made it easier to repel the Italians and reports were given that often hands flew away at the first strike whith an axe or shovel.
It is probably the bloodiest battle fought by a Slovene unit but maybe not the biggest. Anyhow, it made honor to the name of 87th IR.