Ahhhhhhhh Bartok.  I have only been really interested in him for the last 2 years, but I like his music more and more as I listen to it.  First on your list of Bartok should be his string quartets.  The Julliard String Quartet makes the best recording, but there are many good recordings.  There are 6 string quartets in all, and everyone should own the complete set.  A broad range of moods and colors is portrayed in all of these pieces.  He moves from very dark feelings to playful little dances, and the overall effect is mesmerizing.  Some people might have problems with the dissonance at first, but if you just try to experience the sounds and let the music work it's magic, you cannot help but be amazed.
    Real Bartok fans will want all 3 of his piano concertos as well.  NAXOS makes a good version of it, and there is a superb recording by Andras Schiff.  They are all action packed, exciting, and beautiful.  I have a preference for the first concerto, but they are all wonderful.  Being a pianist himself, Bartok wrote lots and lots of great piano music.  There is a great recording on the Pearl label called "Bartok plays Bartok" which gives you an idea of what a talented pianist he was.  The disc is entertaining from start to finish.  There is another disc on the Masterpiece series label that has Bartok playing his piece for piano clarinet and violin called "Contrasts".  Benny Goodman plays the clarinet part.  The remaining time on the CD is filled up by Bartok playing several of his shorter piano pieces from his pedagogical piano series called "Mikrokosmos".  Buy it....buy it now.