Some drinking cups of the Eastern Front

Left to right:

-German interwar paramilitary canteen cup, same as WWI style, holds 300 mL
-Pre-WWII and early war Wehrmacht aluminum canteen cup, holds 375 mL
-WWII Soviet enameled steel cup, holds 380 mL

All of these are metal and can be used to heat water for warm beverages or for shaving.

In the reality of war, German soldiers used Soviet cups and vice-versa.

Postwar or reproduction equivalents of all of these exist.

From a utility perspective, the WWII German canteen cup gets my vote for being most useful, with the best combination of volume and durability. At the time of writing, Hessen Antique has good reproductions for a cheap price.