This guy analyzed the chords of 1,300 popular songs for patterns. And here is what he found:
C (and its relative minor, A) are the most common by far. After that there is a general trend favoring key signatures with less sharps and flats but this is not universal. E♭ with three flats, for instance, is slightly (though not statistically significantly) more common than F with only one flat. B♭ only has two flats but is way at the end of the popularity scale with only 4% of songs using that as the key.
Nashville Number System anyone:
This reminds me of the post I made here – all the most popular songs are basically the same chord progressions.