Maybe for the piano, but I find that C major is a pain for any keyboard instrument (marimba, xylophone, etc.) You aren’t actually touching the instrument and are working from a distance of about a foot. In my experience, having at least one sharp or flat makes life a lot easier, especially in runs. Playing only the “white” keys is harder for me as it’s easier to hit between bars or skip a bar.
In which key is the national anthem of the UK God Save The Queen played? What about other national anthems?
Thanks for that, Floyd.
Glad you enjoyed it. That was the Wendy Carlos version.
On string instruments, “open strings” are the ones that aren’t fretted. If you pluck a string on the guitar with your right hand, while doing nothing with your left, the string is being played open (aka in the open position). If you take your finger from your left hand and press the string to the neck, thus shortening its length and raising the pitch, and then pluck it, it’s a fretted note (i.e. those parallel divisions you see width-wise on the neck of a guitar are called frets.)
Open strings have a more resonant sound then fretted notes. Open chords (chords which generally consist of at least one open string (preferably in the bass) and then other fretted notes sound more resonant then fully fretted chords and require less energy. It only makes sense that rock guitarrists would go for the keys based on the open chords where they can get the most resonant energy while spending the least amount of their own energy.
OK, to those calling me on my assertion that “c is the easiest key on the piano”, your point is well taken. It’s true that the black keys can allow for smoother transitions from note to note based on what the hand tends to physically want to do. And yes, as was mentioned, B major fits under the hand the way a glove fits over a hand playing a B major scale. And yes, many players do enjoy playing in the distant keys with 4 or 5 b’s so they can hang out mostly on the black keys. I have to agree that technically, Maple Leaf Rag in Ab was much easier on the hands than I imagined it would be.
BUT…
The piano is still generally considered a C instrument. From this point of view, every other key is just a transposition of C. C major is the starting point and any teacher should teach the C major scale before the others. It is still the easiest for the young absolute beginner child to play, and more generally, it is the easiest key to “see” theory and relationships with. It’s much easier for me to think in terms of C while composing at the piano then most other keys.
All correct, for the guitar 
Violin-family instruments have their strings ‘stopped’ - they don’t have frets. But that’s a nit-pick.
An important aspect of resonance is that when playing one string, it may or may not cause the other untouched strings to resonate. If it does, and this is most likely when in a closely-related key, there’s far more tone and volume produced. The best example is a top A on the violin, which actually causes all the other strings to resonate, because it has a place in each of their harmonic series. Again, another reason so many violin pieces are written in A major!