I chose 1911, and "other". The 1911 because if you're limited to ball ammo, you're insane to chose anything other than the biggest pistol bullet you can...
For a long-arm of what was actually availible, I'd have wanted a battlefield pickup Stg44 with whatever low-powered optic the Germans were sticking on them. It was really the right rifle for the right war. As great as the M1 was in so many ways, it was the perfect rifle for WWI, and didn't quite reflect the realities of mechanized infantry, and the largely dense countryside (hedgerows), suburban and urban combat in the European theater, and the jungle and hill warfare of the Pacific. A typical case of "Perfecting the tools and tactics to fight the last war better..." Not that America would be alone in that. (The Japanese take the cake, other than the intriguing ballistics of the 6.5 Arisaka round, they were largely armed with Rube-Goldberg crap...)
If our side (America & the Allies) were forward-thinking enough to have come up with their own equivalent of the Stg44, or close, I'd have obviously rather carried that out of loyalty. Perhaps something like Garand's original .260 caliber with a 20 round magazine would be an excellent compromise. Essentially an M14 in 6.5... It would have been much flatter shooting and higher velocity than the 8mm Kurz for sure...
I tend to think that the "sweet spot" for an infantry rifle is at the low end of "real" rifle cartridges, not at the high-end of the intermediates...