If you could become the supreme master of any single musical instrument, even one that you would normally have difficulty with, which would you pick?
Rule #1: It has to be a particular instrument. For example, you couldn’t pick all the different types of trumpets.
Rule #B: Voice and baton are out.
Guitar.
Going to start piano lessons soon, however. Taking up an instrument at 45… why the F not?
Piano.
What type of guitar?
Kazoo. Easy to learn, difficult to master.
But seriously, as a pretty good guitar player who hit “the wall” a long time ago, my initial reaction is to say that, but actually I’d go with the piano.
Piano is, IMHO, is the opposite of the guitar in the respect that it is deceptively hard, where the guitar is deceptively easy. The guitar initially appears to be a very complicated, abstract instrument with all of its difficult fingerings and picking techniques and such, but once you learn few basics it’s pretty easy to pick your way to sounding competent, at the least.
The piano OTOH, it seems like it should be really easy: all of the notes are right in front of you, conveniently arranged from low to high; how hard could it be? It’s hard. REALLY hard. There is so much more theory and knowledge you must learnt before you even begin to sound like a competent piano player it’s a daunting task. I was forced to take lessons as a kid and learned a few basic songs (of which I only remember one; amusingly that one song is “The Muppets Theme”) but all the theory and scales and other technical stuff I could never ever really get a grasp on. After I stopped the piano lessons my father bought me a pawn-shop guitar and I picked it up immediately.
YMMV, of course.
You mean style or just electric vs acoustic?
Guitar would be my choice. Electric. If I could only choose one style it would be becoming the next Stevie Ray Vaughn.
Unfortunately I have proven I have no musical talent.
I’m pretty stuck between guitar and piano. I have very small hands though, so I never could seem to master much of anything on either. Not that I tried too much. I can’t read music at all.
My first and only music teacher told my mom it was hopeless for me. I was a straight A student and could not grasp any part of the lessons. That was in 4th or 5th grade, I think.
Violin, I think. I think it is very rewarding to play with other people, and the violin presents lots of options, from duos to full orchestras to traditional music groups. And, unlike the piano, the violin is highly portable.
Piano. All my rock aspirations disappeared decades ago, so I’d just like the skill to noodle around on the piano in my dotage.
Drums. I can drum a bit, but I’d like to do it to a professional standard.
Trombone, so I could help my two children with their lessons.
Cello.
A B/C Irish button accordion or an Anglo concertina. I’m fascinated by squeezeboxes, but I’m really TERRIBLE at them.
I learned a few tunes on a D/G button accordion, but it doesn’t come naturally to me at all. I have a small English concertina that challenges my brain too.
Electric.
Acoustic guitar. Folky bluesy bluegrassy Americana finger picking acoustic guitar, to be precise.
I’ve played piano for 12 years in August, learning from an amazing mentor/friend as I’ve grown up. I still haven’t mastered it, due to time and lack of having easy access to a true piano. I need to finish my Alfred’s collection of piano books still, then it’s on to what I can get myself to study of the works of the Masters. And a whole ton of other stuff from the collection of sheet music and books I’ve amassed over the years.
I have a small piano keyboard that doesn’t let me have my dynamics; I’ll admit that it’s better than nothing. I’d be overjoyed to just have a digital one with 88 weighted keys, a damper, an una corda and a sostenuto though (something along the lines ofthis). But they ain’t cheap, unfortunately. And I think I’d die of happiness and melt into a puddle on the floor if I ever had a real grand. swoon
But oh, when I’ve got a real career started, I will have at least a lovely digital with pedals and weighted keys. And I will finish my training. My hands and feet are aching just thinking about how awesome that’ll be 
We have one of these to be delivered on Tuesday, a Yamaha Clavinova 430. My daughter takes lessons and it’s time to upgrade from her little keyboard.
Cello.
Drums.
thremin