The usual answers to the OPs question is that the British Lee Enfield was the best, because it had a 10 round magazine and had a bolt that apparently could be worked faster than a Mauser-type bolt. I have not shot a Lee Enfield, so I only know this from books, but British authors seem to agree on it pretty solidly. The 303 round it used was perfectly adequate by WWII rifle standards although the big rim made it a pain in light machine guns. The Number 4 Lee Enfield made during WWII had a nice aperture sight as well (once again, according to books I have read).
The other usual answer is that the Italian Mannlicher Carcano was the worst, because it was chambered for a poor cartridge: the 6.5x52mm, which is said to be inferior to the 6.5mm Arisaka and even more inferior to the 6.5mm Swede. It had an awkward en-bloc clip loading system (sort of like the Garand, but without the big payoff of being a semi-auto, and holding only 6 rounds instead of 8). British writers say a number of other uncomplimentary things about it, such as it having a weak receiver (the phrase "bolt in the brain" sticks in my mind) but again none of which I know about first hand.
The Italians tried to improve the cartridge on the cheap by simply widening the mouth of the 6.5x52mm case to take a 7.35mm bullet. They began making new rifles in this caliber, and converting old ones. Unfortunately, they had waited until about 1938 to start this, and so had not gotten very far by the time they got into the war. They went back to 6.5mm as the standard, and both converted and reconverted some 7.35mm rifles to 6.5mm, and let others just sit throughout the war. Italy was a poor country, and before WWII the Army budget went into things like conquering Ethiopia and backing Nationalist Spain instead of new basic equipment.
PS - I see NIGHTLORD40K has said exactly the same thing as I have, only with 98% few calories.