With plentiful ammo, I?d pick the FN P90, no question. For reference, the FN P90 is a submachine gun that fires the armor-piercing tumbler 5.7x28mm round. It more or less combines the penetration power of the much larger 7.62x39mm round found in the AK-series and just about every Russian weapon on earth, with the remote wounding effects and accuracy of the NATO 5.56x45 round. The P90 goes a step beyond both the AK-series and the AR-15 series by having a total of fifty rounds in the standard magazine, compared to thirty. While the specialized ammo is rare, kits exist to turn the P90 into a much longer ranged semi-automatic rifle with a barrel extension and telescopic sights. The 5.7x28 round also has a flat trajectory and exhibits recoil lower than the greatly inferior 9x19mm submachine gun and pistol round it was designed to replace, while boasting vastly superior performance.
TL

R, it?s one of the best submachine guns in the world, in my opinion, and can be turned into a variety of other weapons with the right modules. The only downside is the rare ammo.
A few other options for me would be the M1 Garand for the massive firepower, accuracy and ease of cleaning, the SPAS-12 for automatic fire and common ammo, the FN Five-seveN for being essentially a pistol-sized semi-automatic P90, and the M1911, for being the most reliable pistol on earth.