What type of German company is best suited for U.S. investors?

If any form of public offering is contemplated now or later, you should consider also to opt for the AG (Aktiengesellschaft - public limited companies). The AG is intended as the appropriate form for entities owned by a larger number of shareholders. Most of our VC clients wouldn't invest in a UG. They would at least changing the form to a GmbH during the investment process.