I'll have to say the 7.92X57(8mm Mauser) as well. It was first introduced in 1888 with a 216gr .318" round nose bullet at a pretty impressive velocity for its day (around 2100-2200fps), loaded in a rimless case when most rifle cartidges of the time were rimmed. When the Germans upgraded to the 154gr .323" Spitz Geschoss (pointed bullet) @ ~2850fps in 1905, it forced the rest of the world to play catch up. It caused the US Army to redesign the .30-03 to the grand old .30-06 (my second pick for "best"). The 7.62X54R (introduced 1891, 208gr round nose @ ~2000fps), although a great and long lived cartridge, was hardly a trend-setter, or unique. Its ballistics were nearly identical with the .303 British (introduced 1888, 215gr round nose @ ~2000fps) and .30-40 Krag (introduced 1892, 220gr round nose @ ~2000fps) of the same era, both rimmed cartridges as well by the way. It wasn't loaded with a pointed bullet until 1908, making it one of the last military cartridges to be so upgraded. They are all great cartidges though, and they all performed their intended purpose (warfare) well.