[drchopchop]

Side note, from a usability perspective: G major is arguably the easiest key to play on a guitar, which is used heavily by both popular music and songwriters. The I/IV/V maps to G, C, D, which are all easy open chords to finger, plus Em is also simple (two fingers).

(Many of the other options include either B major or F major in the I/IV/V, and those are considerably more difficult to play)

[mjr00]

Yeah agreed, G and E make the most sense for guitar for those reasons. Although equal temperament makes all scales sound the same in theory, there are often practical concerns. For example, EDM is very often in D#, E, F, F#, or G minor. There's a good reason for this: sub bass frequencies hit hardest around F. G#, A, A#, B and C sound too high, and anything lower than D# you risk playing on club speakers that can't produce the frequency well.

[taylodl]

The key of G (G,C,D) is really easy to play on guitar and so is the key of C (C,F,G). A lot of beginning guitarists struggle with an F if you play it as a barre chord on the first fret of the low E string. My favorite option for playing an F is to play an FMaj7 using just the D (3), G (2), B (1) and e (open) strings which is an open chord. If you ask me to play an F chord that's what I'm going to play. I've also heard it referred to as the "rock 'n roll" F.

[tshaddox]

D major and A major are the other ubiquitous beginner-friendly keys for guitar. I distinctly remember a few months into getting my first guitar concluding that G major was so much better because Em is so much easier to play than Bm or F#m. :)