If you're only seeing one thing in Antwerp, then I suggest choosing the Plantin-Moretus Museum. This is an amazing museum, amazing enough to be included in UNESCO's World Heritage Sites. I learned a lot in this museum, which focuses on the life and work of 16th-century printers Christophe Plantin and Jan Moretus. There are plenty of cultural treasures in this building, and knowing that life in the 16th century isn't as advanced as life we have now, then this makes the place all the more amazing.
The main focus of this museum is on printing. Plantin and Moretus were known for their printing work, and this played a huge role in advancing human knowledge. The museum houses the two oldest surviving printing presses in the world, and it also has plenty of artifacts on exhibit about typography. If you realize that most people back then didn't even know how to read and write, this is indeed an achievement.
The museum is housed in the former printing press, and you'll need about two hours to fully appreciate what is inside. The pictures above show you the courtyard.
There are plenty of ancient machines here, all having something to do with printing. This is definitely a place to behold, and I spent some time staring at these implements and old machines. It makes me appreciate the books I read. Nowadays a book is very accessible to most of us, and we take it for granted. But back then, it wasn't the case. Books were a luxury, and not everyone could afford one.
The ancient library has plenty of valuable collections. Unfortunately they are all behind closed cabinets, but I would have loved to peruse some of them. There are very interesting books among the collection, including the Plantin Polyglot, a 16th-century Bible in five languages.
The pictures above show the rest of the museum, which also shows other artifacts aside from books and the printing press. If you dig old historical artifacts, then this is a place to discover. It emphasizes the role of Antwerp in European civilization. Establishments like these more or less sealed the importance of the city as a place of knowledge and learning. So if you're in town, by all means try visiting it.