One thing to consider is that the open strings really only matter to guitarists on the less advanced side of the learning curve. Typically, guitarists care about open strings for two things: open chords (you know, two-three fingers and a bit of muting), and harmonics. Open chord voicings are what beginners use the open strings for (not that there's anything wrong with that, though).
Anyway, in G major, the I chord, IV chord, and V chord are G, C, and D. All of those have nice, easy open voicings.
In C major, those same chords are C, F, and G. C and G are fine, but people who play open chord voicings often note that the F major chord is one of the most difficult ones to play. So, at least by the I-IV-V metric, C major's not as easy as G major for open chords.
Actually, D major is another possibility: D, G, and A are all pretty simple (especially compared to F).
E major also is another popular guitar key, for good reason: most guitarists learn pretty early on how to play E, A, and B.
A major, too. A, D, and E are prioritized chords as well.
Not sure whether you could say that G is the best key for guitar. But it certainly ranks pretty highly on the list, especially against C major because of the difficulty of the F major chord. Most chords in G major (not just the I, IV, and V) are pretty easy to play, and by the time a guitarist learns about chords going much farther outside of the key, they can probably play in just about whatever key they so desire.
Keep in mind, also, that this is within a very specific context of generally harmonically simple songs, and in different genres, the easy keys may vary (as an example, Blues, with its 7th chords, would have different entry-level difficulties for its 12 keys).
These are the types of chords I'm talking about, for reference: