I'd find it hard to argue against the United States. Its navy was gargantuan, as was its air force. "Lavishly" is a good way to describe how the US spent on equipping its men. On the ground the US gets criticized for deploying the Sherman and tank destroyers rather than the heavier tanks some other nations produced, but Shermans were produced on such a scale that they were ubiquitous. That meant that there was never a shortage of them, especially for supporting infantry. Toss in the incredible amounts of mechanized transport for the ground forces and the profligate use of American artillery and the US army was very potent. Add in the logistical armada of Liberty ships, a bewildering array of amphibious landing craft, the subtraction of Lend-Lease material, and that the home front never underwent much in the way of privation and I think that the US comes out underrated for its equipment during WWII.

But really, this question can't be answered in an academic fashion. Even comparing just tanks or just aircraft gets complicated by the various outside inputs that affect these items (production capacity, economic cost, design limitations, the desire to innovate contrasted with the need to produce a design immediately, the various personalities involved, the strategic background behind various design decisions--range or speed, guns or armor, quantity or quality). The P-51 Mustang is widely lauded as the best propeller-driven fighter of the war, but it required long range and an expensive powerful (and vulnerable) engine, while other designs could sacrifice fuel capacity for raw performance because they had no need to do long range escort missions. So is the P-51 better than the Fw-190 or the Spitfire? All three were capable fighters and served their nations well and we could spend all day debating which one we'd rather take into combat (and in my younger days I did just that with friends, especially on trips to what is now the National Museum of the United States Air Force), but there's no real objective way to determine that one was better than the others. It can be a fun debate, so long as you understand that there is no real right answer and that the answers supplied by anyone (including myself) are going to ignore the myriad complexities of arms procurement.