In investing, its all about risk vs. reward. For that reason there's generally no such thing as the "best" investment. Different people have different investment goals.
US Savings bonds specifically have a reputation for being the world's safest possible investment, as they are backed by the longest-running sovereign government in the world, and at the time had only had minor technical defaults twice in 200 years*. One would imagine that was rather appealing to a lot of folks coming off of the Great Depression, where banks and companies were dropping like flies, taking their investors with them.
Of course due to that reputation, they don't have to offer a super competitive return. So if you don't mind the extra risk, you can always find a better return elsewhere than US Savings bonds. But if for you the alternative is keeping your life's savings in cash because it's the early 40's and you don't trust institutions, US Savings Bonds were a much better (both safer and better interest) investment than that.
The moral dimension of investing shouldn't be ignored either. There will likely be a world after we go, and it will tend to have more and better of things that we chose to invest in.
Most Americans at the time were not military age men. Investing money in the US government at the time was seen as a very real and effective way for men and women past military age (or otherwise ineligible) to contribute to the war effort, by allowing the government enough resources to keep the fighting men better fed and equipped.
* - In both of those cases, it was a refusal to redeem in gold, as the bonds initially stipulated, not a total default. There was a third incident in 1979 where the payments came late.