There can be no one greatest of all time IMO. However, there can be those who are great because they changed the guitar playing landscape. For me these would be: Les Paul, Jimi Hendrix & Eddie Van Halen. This is not to say that others weren’t doing what they did before them, or did it better. But it’s these guys who influenced a bigger majority, altering the way the instrument was played and/or perceived on the wider stage.
That said, if the greatest guitar player of all time (to date) was someone who could play in all styles and could leave you slack jawed, I’d have to say it’s Guthrie Govan. Obviously, he’s not a household name, even amongst those who play, but he is truly amazing in his technique and wide ranging abilities in so many styles.