Coverdale sounds nowgere near as good as he did when he started out with Deep Purple. His voice these days is pretty shot.
Of all of the singers from that era, I would say that Glenn Hughes's voice has remained in top condition. It is probably even better these days than it was when he was wih Purple.
Well... He was different. Whether he was better or not is all up to opinion. I like his more throaty rock voice from Burn and Stormbringer, when he was a young 21 year old ragamuffin
But listen to him in the modern day and you will see that he is nowhere near as powerful as he was in the 70's or 80's