I’ve played clarinet, alto and bari sax, trumpet and (baritone). The hardest of all was probably alto sax, due to the difficulty in controlling volume and because they had hard parts. This was followed by trumpet, which wasn’t that hard mainly because I was on 4th part. 
I saw Tom Griesgraber (part of Agent 22) playing the Chapman stick here at school once. I gotta say, I was really impressed. Pretty good music, and lots of talent to pull off an instrument like that.
I know nothing about music, but I wouldn’t think the tamborine would be too challenging.
For easiest instrument, I’d have to say the Irish Whistle or Tin Whistle. Of course, to play it beautifully you need a lot of skill, but anyone can pick up a whistle and learn to bang out a few tunes in a matter of a minutes to hours. It’s easier than the recorder, and it’s also easier to get good intonation.
I’ve played saxophone, piano, and guitar. I enjoy playing my whistle more than any of them. Mainly because it’s easy - I have one sitting beside my desk, and even while I’m just waiting for a page to load I’ll pick it up and play a few riffs.
The other nice thing about a whistle is that they are cheap. Usually, a cheap instrument is junk, but with a tin whistle, that’s not the case. There are professional whistle players who play with $10 Clarke whistles.
There are also some very good Whistle resources on the internet, like the Chiff and Fipple forums. Lots and lots of free music as well, because a lot of it is traditional music that is in the public domain.
True. Violin looks hard, but I’m assuming it’s not as hard as I think if there are so many violinists in my string ensemble. In my experience, cello is very easy to pick up, and can play lots of interesting music. I’ve played cello for two years.
No one has mentioned the cowbell?
Not recently.
I started teaching myself cello less than a month ago, on my 70th birthday. It’s not too hard to play (I’ve played violin and viola previously), but hard to play *well.
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One distinction I’ve often thought relevant between “classes” of instrument is whether it requires a special technique to make a decent sound out of it. For example, tap a piano key, or strum a bass/guitar/mando string, tap on a percussion instrument, and you get the sound the instrument is supposed to make. But a bowed instrument requires bowing technique to get an acceptable tone. A wind (other than kazoo/recorder…) requires appropriate embouchre. Sure, tapping a key or strumming a string is a far cry from making music, but I think it adds a different wrinkle of complexity if you need to work just to get a decent sound out of an instrument.
I play upright bass - mostly bluegrass, and I often think my part is so much easier than all the guys who have to play all those notes with all those tiny strings so close together. But driving a solid beat requires a different type of skill.
I also think different folk will vary greatly as to how they rank instruments in terms of difficulty. For example, I’ve always found guitar WAY more difficult than mando, tho I’m not sure why.
This is kind of a “how long is a piece of string” question.
For example, one could say a piano is easy because one just presses a key and a musical note plays, something that anyone could do with no training, while a wind instrument requires a lot of learned technique and practise to play a single note.
However, wind instruments are mostly monophonic (playing one note at a time), while on a piano you’ve got ten-note polyphony (ten fingers to play ten notes at a time). Therefore one can play with greater musical complexity on a piano.
Indeed, with a complicated piano piece one could effectively be playing the musical parts of four or five other instruments at one time. For example, the left hand could be playing the bass line and also a syncopated chordal part. The right hand could be playing the lead melodic line, as well as harmony to the melodic line and on top of that secondary countermelodies or other musical textures.
Of instruments I’ve tried to learn recently, tin whistle has been the easiest though I’m far from going pro. Harmonica is difficult for me to isolate my mouth over the correct location and remember to breath in or out plus I get dizzy. I failed at the didgeridoo spending a couple weeks on it and unable to get much of any sound.
The Great Highland Bagpipe (the most common type of BP) is often claimed to be the most physically difficult to play, not as a matter of skill, but just physical effort.
Depending on how stiff the chanter reed is (and wimpy reeds tend to be very temperamental) you have to blow REALLY hard. Some can’t blow that hard without some training. And you have to do that every single breath, and you have to keep your lips sealed around the blowpipe.
They evolved to be as loud as possible, so went to the limit of what a stout highlander could manage.
In my opinion, the “easiest instrument” and the “most difficult instrument” can vary depending on the way your face is and what is most natural. 