I can't vote, unless the scope of the question is narrowed a bit.
Best in terms of production cost and time?
Best in terms of battlefield performance?
Best in terms of reliability?
They are both good designs, but they both have their shortcomings and strengths.
The Mosin is a better rifle for arming massive armies of uneducated conscripts.
The Mauser has a better home in the hands of a precision shooter, or civilian.
"Such is the strange way that man works -- first he virtually destroys a species and then does everything in his power to restore it."