My candidates;

1) M1903 Springfield. They should have copied the 98 Mauser when they designed the safety and two piece firing pin. The '03s before the A3 version had sights more refined than a combat rifle needs. Then there was the early brittle receivers. The barrels were not stepped like the 98 Mauser, causing vertical stringing when the barrel got hot.

2) G3/HK91 family of rifles. I hate the funky charging handle up on the handguard area, and the other reasons already listed. I shot one during a US Army-Bundeswehr "partnership" range in the late 1980s, and IIRC, they don't have a bolt hold up feature after the last round is fired.

3) The Vz58. Another over-engineered and over complicated gun from central Europe. In wartime these assault weapons would be too labor intensive for rapid production. The magazines are not compatible with the AK family of weapons. I examined some captured Vz58s, found that they are a son of a gun to reassemble....it takes three hands to get the recoil springs and bolt back in the gun.

I'd like to make a few comments about the Garand. Firstly, there is NO documented evidence that the pinging clip ejected from a Garand ever gave any Axis soldier the hint that the GI had to reload his weapon. Official unit histories do not mention this "shortcoming". People have researched the diaries, letters, and memoirs of Japanese and German veterans who faced GIs and there is NO mention to substantiate the assumption that they waited for the tell-tale "ping' to make their rush on an American fighting position. Likewise, there is NO mention in any Axis training manuals about exploiting the magazine clip ping as a heads up to attack a reloading American. If you examine American popular culture (movies, magazine articles, etc...) there are NO wartime references to this as being a tactical shortcoming. Could the tell-tale "ping" story be an urban legend??? I doubt that a German or Japanese soldier could hear the ping of a Garand clip over the din of battle. Since a Garand reload could be completed in less than 3 seconds by a trained GI, I think the issue totally irrelevant.
The succes of the Guadalcanal Campaign is a testament to the bravery and training of the Marines that fought there, not a justification of the M1903 Springfield. One has to wonder how many brave Marines died or were badly wounded during the begining of the campaign because they lacked the additional firepower of a Garand. One thing is for certain, the Marines on Guadalcanal couldn't get rid of their '03s quick enough when the Garand became available. As a matter of fact, the Marines routinely "appropriated" M1s from the follow-on US Army troops. BTW, the Japanese failed on Guadalcanal because their navy found it increasingly difficult to resupply/re-enforce the troops they had there. Disease and starvation forced the Japanese forces to finally evacuate the island. There is a historical urban legend that the Japanese were very much at home in the jungle, but the reality is that the Japanese had even less experience fighting in jungles than the Americans. Due to this factor and a poor logistics system, the Japanese fighting men suffered terribly from from tropical diseases, lack of potable water, infection of wounds, torment from insects, etc, etc... The banzai charge wasn't really a tactic, it was a suicidal gesture used as a last resort to secure an honorable death when defeat was imminent. The Japanese already been schooled during the Russo-Japanese War of 1904-05 as to the futility of launching frenzied bayonet charges against defensive positions.

It doesn't make any sense from a statistical standpoint to assume the carbine inflicted more casualties than the Garand in WWII. The Garand was issued to more infantry soldiers than the carbine, and infantry soldiers were more apt to engage enemy troops with their issued weapon than combat support and service and support guys. The original rationale for the carbine was that it would be a more effective weapon than the M1911A1 pistol. Therefore personnel that might have been issued a pistol got the carbine...company grade officers, jeep drivers, Signal Corps troops, MPs, etc... Likewise, specialised troops who would have been issued a short rifle or carbine in the 19th century (artillerymen, combat engineers) usually got the carbine. Not only did a larger percentage of frontline troops receive the Garand, ballistically the Garand is potentially twice as deadly than the carbine if we consider the maximum effective ranges for each weapon (300 yards vs 600 yards) and the greater weight of the 30-06 bullet (more tissue/bone damage).

I checked my various reference books and also waded through 6 pages of Goggle entries for "Garand, casualties", and I couldn't find any quantitative data. The USMC and the Army evidently never ran a scoreboard to track Germans and Japanese killed by specific small arms. Obviously the Germans and Japanese had much more to worry about than tracking how many of their guys were killed by M1 carbines versus Garands. IMHO it would have been an impossible task for either side to keep a tally, that is without keeping scrupulously accurate counts of casualties and doing autopsies on all the dead guys to determine who got shot by what weapon.