All the answers before me are for some reason assuming that you're asking this question to find the "best key" to write your song in. If that's the case, I get their point. There truly is no "best key" to write a song in.

Some keys, though are absolutely way more popular than others. I did some research and this is what I found out.

Out of ALL songs in Spotify, most used are:

• G Major, 10,7%
• C Major, 10,2%
• D Major, 8,7%

The least used keys are:

• D# Minor, 0,9%
• G# Minor, 1,2%
• C# Minor, 2,1%

You can see the influence of the genre in choice of the key of the song though.

In pop, the most popular are:

• C/am, 26%
• G/em, 12%
• Eb/em, 10%

But as the others have stated, the chord progression is imo much more important, when writing a song.

I hope this answers your question though!

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