My choice

My preference is the M1 Garand rifle, closely followed by the Mauser.

Altough the Mauser is the best in certain categories, the M1 is the best overall rifle. The 8 rounds en-bloc clip and the firepower that comes with a semi-automatic rifle make it a winner.

Moreover the lower recoil helps long term use.

If I was to select a rifle as a base for a sniper rifle, then the Mauser would be my choice.