Flight
Best international airport to access the city. 20 minutes by train to the center. Buy your Zaventem-to-Midi train tickets at the kiosk in the station when your plane arrives. There is also Brussels Charleroi for flights from within Europe, but we would avoid if possible as it's less convenient to access from the city.
Flight
Coming from the US, you might find a better deal on plane tickets if you fly into Amsterdam. There are two train options to Brussels from this airport: the Eurostar in 1.5 hours or the local Belgian train (eurocity direct) in 2 hours (which would be cheaper). Best to book your train tickets in advance for this option.
Flight
Paris is another option to fly into. Trains run regularly from Paris to Midi, 1.5 hours usually. Also best to book train tickets in advance if you go for this option.
Train
The main international train station in Brussels. The train from the airport will also arrive here.