Its hard to argue against Elvis as the best pop male vocalist of all time. Guys like Andy Williams, Sinatra, etc. come close, but I don't think get there. As much as many of us may not care for his music, you can't say Barry Manilow isn't near the top of this list. He doesn't sing as well in recent years as he did when he was younger, but his vocal tone and range were really good back in the day.
I'm not sure Sammy Hagar had the best sounding voice, but he had one of the better ranges. Contrary to popular myth, Van Halen was far improved with Hagar -- not just vocally but musically as well.
For country and even a little bit of pop, Glen Campbell. I'd put him right under Elvis.