The most appropriate type of company to open in America depends on the activity you want to engage in.
Anyway, with a bit of realism, we can say that the types of companies Italian residents can open in the United States are two: Corporation or LLC. Or rather: C-corporation since the S-corporation is reserved only for American citizens or permanent residents [i.e., green card holders]. There isn't a single best company type universally valid for the United States, a form that can disregard the state where you want to open the company and the company's operations.
Choosing the most appropriate company type for the USA also depends on variables that are foreign to purely operational considerations, such as the need to plan a U.S. visa application or tax optimization strategies.
Below, we propose a very general "company type <=> activity type" correspondence, emphasizing that each situation is unique with its own needs and particular circumstances. The proposed list, although generic and generalist, highlights some variables that lead to preferring one form of company over another.
We also remind that an important, sometimes decisive, element in choosing the form of company to open in America is the American state where it needs to be opened. The following list disregards these considerations.