The Concerto for two String Orchestras, piano, and tympani (as mentioned above) is the masterpiece, followed closely by the Symphony #3. Alas, Martinu wrote a ton of music, and most of it (as it also true of Villa-Lobos, and Milhaud, other hyperproductive composers of the twentieth century) is workmanlike and entertaining rather than profound, falling rather too easily into formula. You could try some of the piano music or piano concertos played by Firkusny, and the string quartets. The opera "The Greek Passion" is also quite fine.
Bohuslav Martinu
Could some of the cognescenti please make some recommendations for his music? I have some of his chamber music and would like to enjoy some of his symphonies and similar works for larger ensembles. Of course, good sound is a welcome feature so you might want to rate them for performance and sound...Thanks, IJ