ゲンナジー・ゴロフキン
There are currently no undisputed champions in this era. Although Bernard Hopkins and Jermain Taylor held all 4 titles, the WBO at that time was not yet recognized as a major sanctioning body. The closest was Gennady Golovkin and Canelo Álvarez; they simultaneously held the WBA, WBC, and IBF titles at one point in their careers.