Fantastic Pompeii exhibit and permanent collection. The other pavilions, especially the ones on the other side of the street tend to be far emptier, so if you're looking for some peace and quiet, those pavilions might be good options. On a Saturday noon, the main exhibit and some of the permanent collections in the main building and on the upper floors tended to be quite crowded.
In particular, the special exhibit (for me, it was Pompeii) was very crowded since there were many guided tours going on at the same time.