Glenn voice is GREAT. No doubt about that. He is groundbreaking in hardrock history. One of the pioneers of the genre. Listen to Trapeze, Purple, Sabbath, Hughes Thrall and his solo work. Great stuff! But what makes him very special is the fact that he still is a superb singer (also live). Together with RJD (RIP). They are the only two hardrock singers who have the same high level as in te seventies. In fact Glenn's voice has become beter and stronger through the years.
(Gillan, Coverdale, Plant etc. still good singers, but they can't do it live anymore on that old high level). Glenn still does and that makes him the true VOICE OF ROCK