My first wish would be the ability of elemental control. This is the ability to control the four elements: water, fire, earth, and air.
The above image was taken from Wikia.
The above image was taken from Wikia.
The above image was taken from Playbuzz.
If I were to be granted this wish, I would mostly play around with it for fun. I would also see if I could do anything major with it, such as control the weather.