Great as he was, it's odd to see Sinatra in a lead singers list. I'm not a Lady Gaga fan but she does an album with Tony Bennett that's pretty good.
Gaga really has great pipes, she's not just a pop show like Madonna. I know Sinatra wouldn't make most list, but his voice was unique - very rich with super cool phrasing. When he got very old it didn't sound as good - too much cigs & alcohol.
Not a fan at all, but ole girl can sing her ass off. I know most hardcore Metallica fans hated it, but I loved the duet/harmony she sang with James on 'Moth into the Flame' at the Grammys.