Harmonica by far. The harmonica is said to be the easiest instrument to learn and the hardest to master. You can learn about scales, chords, melody, rhythm. Its simple to play, early success keeps you interested. Its small and portable, you can play it anywhere. It's a very versatile instrument in terms of music types. Its much more than blues and country tunes. I have many CDs of bluegrass, traditional Irish, Klezmer, Zydeco, Jazz and Classical music.