Which of the motives for American imperialism do you think was the most important?
I would argue that the most important of the motives for American imperialism was the desire for military power and national prestige. This motive was more powerful, I believe, than the desire for commerce or the desire to “take up the white man’s burden.”
There were surely those who felt that it was very important to “civilize” the rest of the world. However, I am very skeptical of the idea that this was sufficient to cause them to undertake things like the Spanish-American War. I do not believe that the urge to altruism was that strong, particularly in view of how the US actually ran the possessions that it took.
Commercial motives are more plausible. Money is a very powerful motivator. However, some of the things that the US did in its imperial period were of dubious commercial value. The protracted war to take the Philippines falls in this category.
That leaves only military power and national prestige. This was a time when men like Theodore Roosevelt would talk about the need for a war to prove America’s masculinity. It was a time when the US was striving to become a world power for the first time. Alfred Thayer Mahan’s work was enormously influential in making the US want a way to project naval power around the world. I would argue that the desire for power and prestige is at least as strong as the desire for money (at least for a country) and that the things the US did in its imperial phase were more clearly aimed at power than at wealth.