"Best" is a highly subjective term - what's best for one use is rarely best for another. .50BMG bolt guns are fun to shoot and can be made very accurate, but it's a seriously big round that requires a seriously big rifle. That's fine for the guy who only has to lug the rifle from the truck to the shooting platform, but for soldiers it's a pain to hump and jump a fifty, so it's got to have some important advantage to the mission at hand. For .50BMG, that's often the payload capability you get from Mk211, which will do far more damage to hard targets at distance than just about anything else out there. Then there's the M82/107 that even soccer moms know about, which in reality shouldn't be classified as a precision rifle as it has accuracy more akin to a battle rifle. That said, for the solider overwatching a checkpoint and facing a VBIED threat, it's a nearly perfect weapon system.
For shooting paper and steel as far as accurately possible, the current king of the hill is .375 Cheytac, a necked down version of the .408Cheytac that stays supersonic out to 2800 yards. Most .375CT rifles are made by the custom shops on any number of actions and stocks to the shooter's specs. DTA is one of the only companies making factory .375 rifles (though the new Cheytac may be actually putting out rifles as well).
But if you're new to precision rifles, you shouldn't be looking at either of these big bore rifles and more at a basic short action .308WIN or 6.5 variant.