I don't think there is a best. It would be terrible if everyone played the same thing. I think the best thing is to let children try every possible different kind. Unfortunately, this isn't practicable for most people.

My first instrument was the piano and I had very traditional music lessons for the first five or six years. It gave me a good basis for some things but playing by ear was discouraged, which was terrible. Also, you don't have to tune a piano yourself --- and can't, and you have no influence over the intonation, as with a violin, so you can't develop your ear very well with just the piano.

I think the guitar is a very good choice. On the other hand, when I was young, a plague of people strumming guitars covered the face of the land and swallowed up every crop and all the fruits of the trees. More recently, there's been a plague of ukeleles. I love the guitar and quite like the ukelele but like I said, it's terrible when everyone plays the same thing the same way.