My vote is for the Mauser. For 50+ years for most armies of the world, it was the rifle to own. It was copied, plagiarized, imitated multiple times (US M1903 Springfield, Arisaka, Enfield Pattern 14, US M1917, etc). Mausers were available in numerous configurations and in many degrees of finish, from 1871 black powder cartridge rifles that dominated the battelefield for a decade, the 1893 that you could hand to any peasant conscript and expect it to perform well, the 1895, the 1894, and 1896 Mausers that were essentially an improved 1893 Mauser, the Gewehr 1898 that was respected on the battlefields of the world for 20 years due to its ruggedness and accuracy, to the 1949 Persian Mauser that was made of modern steels with a fine finish on both the wood and the metal. These rifles were still standard issue for many armies well into the era of autoloading weapons.
Not that the others were not good rifles, just that the Mauser design stood head and shoulders above the others. The larger capacity of the Lee Enfields gave them greater capacity but made them slower to load. The Mosin Nagant was somewhat crude but could be relied on to perform well in any conditions. The Springfield was the sweetheart of many WW1 fans, but the US M1917 did the yeoman's duty for the US in WW1.
And of course, the "also rans" of WW1 had battle rifles as well. The French had the Berthier and older Lebel rifles, Italians had the 1891 Mannlicher-Carcano, Austria-Hungary, Greece, and the Dutch had the M95 Steyr, Turkey had both 1893 and 1898 pattern Mauser rifles of varying designation and sources, Serbia armed its troops with Mauser 1895 pattern rifles, Albania was armed with Steyr M90, Martini-Henry, and Winchester 1873 rifles. Many nations, many rifles. It's not as easy as Germany/Great Britain/USA.