The Digital Dream Door music forum -- I have no idea who they are -- has generated a list of the top 100 most-skilled male rock vocalists. They place Adam second behind Jackie Wilson. There are 332 pages of comments on the thread wherein people debate the list's contents and order. It's, of course, an impossible silly task to rank musicians, as if what they do is quantifiable. It's fun reading, nevertheless, and they do work really hard to stay focused on the technical aspects of singing. Lots of links to performance video and audio are included so it's a treasure trove of great vocal performances.
There's a point by point breakdown comparing Adam and Freddie here. (H/t to Kate Ewing (aka Nolechica on Twitter) for the link to the list and h/t to Marjorie for getting me to the vicinity of the point by point discussion.)
Update: I see that by the time they get to page 331 in the comments, the group has changed the order and Adam is lower. But still in the top few no matter how you cut it.