I'm surprised noone has mentioned vocals?  I mean, the human voice and everything that goes along with it is so versatile!  From Mongolian tonal singing to beatboxing, from singing solo to singing in a choir, you have a hardwired instrument between your lips!

The thing with vocals is that it's very easy to sing the wrong way and damage them permanently. If you damage a piano, guitar or violin, you can just go out and get a new one, but replacing a set of vocals is something else. Even people who have a natural talent for singing can ruin their vocals if they are not careful. Therefore, I really wouldn't qualify that instrument as easy. The art of proper breathing, projection to minimize the strain on the vocals, and maintaining the correct posture is key to mastering it, and this I know from many years in the choir and doing solo gigs.

Hi there,
@1: I would say nerves. I am naturally very good at playing on them.

But seriously, when it comes to easiest instruments, excluding the really simple like your thighs or your bottle, I would say guitar and flute.

It's true, that guitar can be quite complex to play more sophisticated things, but for many modern songs, you'll be just fine with few chords in your hand and few days of practice. The only problem is, that holding the strings will be kind of painful from start, if you have soft fingers like me, I don't play guitar for this reason.
Flute is quite easy instrument because it has no chords, and most well-known melodies for it are slow. I don't recall any song, which I wouldn't be able to play on it after 30 minutes of searching the right holes to block.
And even that is for full-lenght song, simple themes, such as My heard will go on, Hymn to the sea, or Perinbaba theme can be self-taught in really just few minutes, real experience.

Now, as for the easiest instruments, which are actually interesting, that's another story.
I recommend accordion. The instrument has a very nice and full sound, and you can play simple things quite easily even without a teacher.
You won't be probably able to play faster things like Despacito without practice, as vertical keyboards is kind of unusual even if you have experiences with piano, but because modern accordions have a wide variety of chords playable simply by pressing buttons with your left hand, you should be able to apply creativity and create interesting and good-sounding songs even without long practice.
I had one some time ago and in an evening, i was able to play few of my favourite piano songs in a very interesting accordion arrangement, just by trying and playing with the thing.
My instrument was rather simple, with only two columns of buttons, modern acordeons have three or four columns, plus switchable registers, so you have a wide variety of tones to play with.

Also, @violin and bass time travellers: even if bass players practiced 29 hours per day more than violin players, don't forget, that they change direction of bow and position of fingers 4 or 8 times less than violinists. And because it is not hard to manage a tone, but to change it correctly and in time, this is like just 15 hours of practice per day. The rest is just holding a tone, even a trained monkey can do that.

Best regards

Rastislav

Btw, how did the idea that bass requires more practice than violin even appear? Is there some kind of joke that I'm missing?

Best regards

Rastislav

Rastislav Kiss It's a youtube thing, I've linked a video below

https://youtu.be/yN8HnXEYIJI