What does 'the best' mean?
Best design? Best shooter? Best loved? Best economical value? Best firepower? Best impact for its day?"
Bolt actions have potential to eclipse semis in accuracy and are in general stronger
In terms of design, I vote for the M1 rifle. If you ask me, the bolt design on the AK47 is similar enough to make me wonder. I recently heard 'negative' aspects of the M1 such as 'the "gun oil" on the M1 used in the battle of the bulge made the riflers freeze up in low temperature'. Any rifle using that "gun oil' then, logically had the same failures, so I discount the entire episode.
Also many rifles were fine designs for their time, and some, like the StG 44 transcended their era. The M1 did also. But I've never seen any 'milsurp sales' include an StG 44 so my vote here goes to the M1
Accuracy- the M1 was and is good in terms of accuracy; Very effective sniper's models were made from it. The same is true of the Mosin nagant, LE, K98, etc, so the M1 loses no footing there. But the M1 is also semi-auto, which gives it the edge when used in other roles different from sniping, so my vote goes to the M1 for its additional usefulness
Each rifle was loved by its users; that's strictly preference, so this is a wash
Best economical value? The M1 was costly compared to a bolt action no doubt. Other rifles that had been in use for a long time (K98, LE, Mosin Nagant) must have been a better value for their country in terms of dollars per fighting man equipped. Today M1 rifles are relatively expensive. So I give the edge to the Mosin nagant here
Some bolt actions had more rounds than the M1, and the US used a rifle with more capacity using the same round (BAR) as the M1 so firepower is an argument. However the BAR was not the basic infantry rifle, and the M1 loses out by only a few rounds compared to the ten round bolt actions. The semi-auto rifle is inherently quicker to fire than a bolt action, so I call that one a toss-up
Best impact? For me, the M1 is my choice. It was used through Korea to the beginning of Vietnam. My opinion is that it may have influenced the AK47 design as well. But in terms of contemporary impact- the USA was the first nation to equip its soldiers with a semi-auto rifle as a standard infantry weapon and that was with the M1. That is quite an impact on the battlefield
I vote M1 rifle