Wlad is higher ranked ATG - cos he is a long-term Champ with great consistency. Bowe has 2 great wins VS a ATG in Holyfield, But was not a v.good Champ.
Bowe's title reign was too short lived and far from glorious apart from the fights against Holyfield. Successful title defences over Ferguson and Dokes are not the credentials of a ATG championship reign. That combined with giving up the WBC belt to avoid facing his number 1 contender greatly diminish his standing. Wlad's reign has also been far from glorious but he has faced the best that were willing and available and he has been champion for about 5 years so that gives him a clear edge, in terms of longevity and dominance.
I rank Bowe over Vitali, but Wlad over Bowe. The depth of Wlad's resume compared to Bowe's makes this Wlad by a clear margin.
Both Klitschkos are higher up the pecking order than Bowe. Riddick's one and only claim to fame is the scoring two out of three in his trilogy with Evander Holyfield. I'd rank Vitali and Wladimir in the lower reaches of the top 20.