The museum of Fine Arts contains the Oldmaster's museum, the Magritte Museum, the Fin de Siècle Museum, the Modern Museum, the Wiertz Museum and the Meunier Museum. All of these expositions are accessible via the same beautiful entrance hall (except for the Wiertz Museum and the Meunier Museum - which are free by the way), but you can choose which part you'd like to visit and buy a ticket accordingly, or get the €15 combination ticket if you'd like to visit several. This combination ticket is valid for one day only, so make sure you reserve enough time to adequately visit the museums.
Below is a little overview of the museums I have visited so far. There is a tremendous amount of artworks on display, so I'll have to return to see the rest.
You might want to start your visit in the Oldmaster's Museum, since this one has the most famous paintings. The collection includes works by old masters, mostly Flemish, like Brueghel, Ruebens, Bosch and Van der Weyden. It contains iconic paintings like the Fall of Icarus and The Death of Marat painted by Jacques Louis David.
The Magritte modern art museum is a walk away and features some of Magritte's lesser known works. It doesn't contain his most famous paintings, but it tells you a lot about Magritte's life. Some other surrealists are on display here as well.
Fin de Siècle Museum
The Fin-de-Siècle Museum showcases international art forms popular in Europe around the turn of the 20th century. It has a nice collection of impressionists and wonderful art nouveau furniture and vases.