Guitar is the easiest instrument with which to make pleasant, rhythmic sound. It is also, unfortunately, an instrument from which you learn the least (or at least most slowly) about music.
Piano is the most versatile for playing and perfect for actually learning how music works. The initial learning curve is longer and steeper, but once passed it, a very wide variety of music becomes accessible. Knowledge gained from learning piano easily transfers to other instruments, despite that the methods of sound production may be entirely different.
Violin is generally considered to be the hardest to learn.