What is the most difficult musical instrument to play?
bagpipes.
Heck, they’re ALL pretty damn easy. Take it from me.
Or did you mean play WELL?
bagpipes.
I’d guess bagpipes, too although the toughest one I’ve ever known anyone to play was the oboe.
The double reeds (oboe, bassoon, etc.) are widely regarded as some of the more difficult instruments to play.
I would tend to say that playing a full blown pipe organ (the original music synthesizer) would be a bit of a rum go. The pedal work in combination with multiple keyboard manuals and many dozens of stops make them a rather demanding instrument to play well.
The Bach trumpet (think of the solo in the Beatles’ “Penny Lane”) is reputed to be a devil as well.
Technically, the one instrument that is played without touching it, the Theramin, has had the fewest recognized masters (2-3) in its admittedly shorter history. Anyone have cites on this instrument’s difficulty? It’s supposed to be a bear.
As the proud owner of a theremin, I can tell you that playing it is easy (you just wave your hand around), but playing it well is a bitch.
The distance your hand must be from the pitch wire changes based on the size of the room, the amount of humidity in the air, etc.
In other words, depending on where you play it, your technique could completely change.
The only world-clast Thereminist I know of is Clara Rockmore who worked closely with Leon Theremin when he invented it and is still around.
My vote for most difficult instrument is Jean-Michel Jarre’s laser harp, a harp with laser beams that the finger interrupts instead of strings. Talk about guesswork!
Bagpipes do get my vote for most dangerous instrument. Piper’s lung, anyone?
but of the wind instruments one might normally find in an orchestra, there are two schools of thought, according to every music director and professor I ever had.
French Horn: It requires the most precise embouchure to produce the proper tones. In addition, the exact placement of the instrument and your hands on the insturment can dramatically effect the quality of the tones. Now keeping that in mind, you’ve got all those pesky keys to work with.
Trombone: Unlike other instruments, there aren’t any keys or valves or fingerings that you can rely on to produce a particular note. Each position of the slide can produce a variety of notes and not all notes have one appropriate position. The positions vary according to the note you’re trying to play, your embouchure on any particular day, the temperature, whether your slide is clean and can vary widely from instrument to instrument. It’s very easy to be out of tune on a trombone. For example, an e natural is played in 2nd position, a high e or g in “raised” 2nd. On my C trombone, these are closer to the bell than on my F horn, but not by much. If you’re off a fraction of an inch, it’ll sound wrong - and you can hear it. I’m biased in favor of the trombone, btw. 
Becoming proficient with an instrument, and becoming truly skilled are two different things. There are a lot of lightweights puffing on clarinets or sawing away at violins. This is all subjective, of course - heck, it’s music. One of the indicators you might want to look at is the program of any high-school band concert. You’ll notice a deficiency of oboe (any double-reed), french horn, trombone (or any low brass), cello. What will you see lots of? Violins, flutes, clarinets, trumpets, percussion, and the blasphemous saxophone. (just kidding sax players). These intrsuments are generally easier to get up to speed on. They’re also less expensive and eminently portable.
I have to agree with the horn as well. I’d also like to mention that it’s pretty hard to pick an octave. I can always remember sitting through sectionals waiting for the horns to figure out their part. But I’ll give them credit, a well played horn IMHO is one of the most beautiful sounding instruments ever.
You may want to take a look at the thread most difficult musical instrument to learn and/or play? over in GD. There are several contenders there, including oboe and french horn. It is a good read.
Well, the “moved threads” button was no piece of cake.
Off to IMHO.
Wow, I’ve got it easy. I only play drums. I must say though that Jimmy Page must be recognized, if not as a virtuoso on the theramim, then at least as the one who brought the greatest amount of public awareness to the instrument, via his use of it in “Whole Lotta Love.”
I think it’s clear that you were referring to a drum set, but try playing the tablas and you might want to come back and update the thread.
(BTW, I have no clue how to play tablas. It is just my understanding from hearing and seeing them played [played well that is] that it is (or is it “they are”?) a particularly difficult instrument to learn).