In terms of accomplishments by a striker who's now in MMA - Overeem and Spong. And relative to his/her own competition - Holly Holm, in terms of both accomplishments and technique.
Inside the Octagon and during actual MMA fights - Anderson Silva and several others. I'm not a fan of Conor, but even Conor has a better claim inside the Octagon. Wonderboy has spectacular offense but he got dropped by both Matt Brown and Jake Ellenberger - who, at the time, were a non-top 10 fighter and the #9 fighter, respectively.