If you could master any musical instrument (with virtually no effort), which one would it be and why?

Piano, because I just love how it sounds and to be able to create that for myself anytime would be wonderful.

I was going to say saxophone, because of its sounds, but @SergeantQueen you beat me to it. It’s the most versatile instrument and it can carry a melody.

When I was a kid I tried to learn french horn, but I was never any good at it, and it was never a melody instrument. It was boring as hell.

Gong, or more precisely, dinner gong.
Far more refined than summoning your food by rattling a tin triangle.

Cello

Voice.

Not necessarily operatic, but clear, strong, gracefully melodic, and beautiful. I have always longed to sing, but have to content myself with doing it alone in my car.

I was envious when the three Disney fairies gave that gift to Sleeping Beauty.

Electric Guitar.
My father paid for me to have one, under the condition that I would get lessons.
So I did take them, but it turned out I didn’t have talent (and I lacked discipline).
Love the sound and feeling of it though.

It’s a toss up between banjo and bagpipes. I’ve always loved both of those instruments.

As an orchestral imperative I would select the kazoo, not only for the complexity of the fingering but also for the delicacy of the embouchure.

I give you the William Tell Overture.

https://www.youtube.com/watch?v=oau9gtG5Om8

I can’t sing at all, it would be nice to have that ability. I’m already proficient with guitar and a few other instruments.

Voice.

Only instrument that I’ll always have on hand!