I'd put Madonna in the Gwen Stefani, Kylie Minogue like group of singers.
Entertainers but not all time great singers, potentially put more effort into the choreography than the singing at times which is why Madonna has lip synched in the past.
True greats don't lip synch.
That's why I put the likes of Ann Wilson above them.
Incredible voice, sold records and most importantly capable of an amazing live performance.
Stefani and Minogue aren't even in the same universe as Madonna. Madge was a global cultural pioneering mega star, and whilst vocally may not have been the best, created timeless scene leading pop music through at least 3 decades.