Wow, best of all time? Eesh... that is very difficult. It really depends on the genre. On the women's side, for me, it is a tie between Aretha Franklin and Dionne Warwick. There is nobody close regardless of genre on the women's side, IMHO.
On the men's side... it's way more difficult for me. In rock, my top two would be Rob Halford and Freddie Mercury. In R&B/Soul, Marvin Gaye, Smokey Robinson, and Ray Charles. In pop, Michael Jackson and George Michael. Unlike the women, I just can't pick an outright winner for the men.
Absolutely, Ann Wilson is phenomenal! Only 1 woman in my opinion is her equal and that's Chrissy Hynde: