The most commonly used airports for such a trip are:
- Frankfurt International Airport (FRA)
- Dusseldorf International Airport (DUS)
Both are served by US-American Airlines from the US. There are also non-stop flights to Frankfurt, which you may want to prefer.
Airport Cologne-Bonn (CGN) is actually closer, but less commonly used for such trips. Trying to use it for a transatlantic trip typically leads to higher airfares in my experience.