I like Veletrzni museum. It’s a place I return to every couple of months. The space is big, with an open central area, well lit galleries, and the curators have used some unusual background colors to showcase the artwork.
Modern art is its mandate, which includes Impressionism through … um … whatever today’s stuff is called. Post-post Modernism?? Perhaps.
The work is primarily from Czech artists. Veletrzni Palace is part of the National Gallery consortium of museums scattered around Prague (and the country, actually). There are areas that have some French masters, and Spanish (Picasso & Goya). Otherwise, you will find Czech contributions to the major movements.
There are some real beauties here, and some avant-garde work for its time. A couple hours is about what you need to get the major works (without racing through the building). If you really stroll, and have an art companion, then your time is extra-well spent. Here are some samples, with Asia and I having some controlled fun in the arthouse. In a day or so I’ll have lots on Flickr.