There isn’t one. Every woodwind instrument has their own difficulties. To give a quick summary: The flute requires the most air and air support. Many beginner flute players tend to feel dizzy at first. Air support is extremely important because many flute fingerings are the same, so a faster or slower air stream determines the pitch you are trying to achieve. The clarinet has the most challenging fingering system. Once in the altissimo (highest) register, there are many alternate fingerings that must be learned. Good tone production can be difficult for saxophone players because the instrument itself tends to be a naturally loud one. It takes time and dedication to achieve a medium loud, smooth sound in all ranges.