There are numerous options for flights to Belgium. The best airlines to book from the US are American, Delta, and United for direct flights to Brussels from New York and Los Angeles. These airlines have code-share agreements with Air France, Brussels Airlines, KLM and Lufthansa so it makes sense to book on your favorite carrier to ensure you get your travel points.
Always check the baggage policies for each airline you are flying on – not the airline you booked on – as the baggage policies are not consistent between airlines, even with the code share agreements.
(NOTE: Due to COVID-19, airlines and routes may change frequently)