This is a difficult question to answer because every child is different. Some may have an easier time with the chordal, theoretical aspects of the piano while others may be more interested in developing an embouchure and playing a brass instrument or woodwind. Given that piano and guitar are chordal instruments and wind instruments are single-line (only playing one-note at a time) makes a big difference. Some kids really gravitate to playing chords and others do not. Some kids are going to enjoy playing the saxophone but others might prefer trombone. The possibilities are endless. One thing I do know, though, is that the two instruments with the biggest learning curves are the oboe and the trumpet. So, those two can be the most frustrating for students initially, especially if they are not taught properly. However, there is no one single "best" instrument for every child.