Angus is an awesome guitar player. Maybe not the best ever, but real good either way. Now, if he had said CC DeVille....
Gilmore has one of the best guitar tones. His playing is very good, his ear for guitar tone is great.
There is no such thing as a greatest guitar player. Only opinions. It is very subjective.
You're right about it being subjective to a certain extent. I love Angus Young, but he isn't great under any metric. He said the best ever, living or dead. While who's the best might not ever be determined, figuring out who ain't should be quite easy, at least till it narrows down quite a bit.