TS specified all time in the OP so here goes my list with that taken into consideration:
HWT - Mark Coleman. Not only the best credentials but also the most dominant heavyweight with regards to using his wrestling in MMA. He had absolutely no other tools besides wrestling and ground n pound and he did quite well for himself during the early days of the sport.
LHW - Jon Jones. His wrestling is so good he doesn't even have to use it anymore. He's toyed with national level wrestlers like Brandon Vera. Randy Coture gets an honorable mention here; his wrestling was better than his contemporaries Tito and Chuck.
MW - Chael Sonnen wins this in a weight class that historically has been pretty thin on wrestling talent. Weidman may prove to be a better wrestler as it relates to MMA as his career progresses and Dan Henderson was had better credentials but chooses to keep the fight standing so he can 'Bisping' his oppponents. Chael gets the nod here.
WW - GSP. To be the best wrestler (by far) in the most wrestling-centric division of the sport makes him that much more impressive. Honorable mention goes to Matt Hughes for his years of dominance with no striking game to speak of. Ben Askren could find himself ahead of Matt Hughes if he were to ever come to the UFC and be dominant; I have my doubts though after seeing him barely squeak by Jay Hieron.
LW - Sean Sherk. He gets my vote by the slimmest of margins in the second most wrestling-centric division of the sport. Gray Maynard, Frankie Edgar, and maybe even in the future, Michael Chandler could surpass him depending on how their careers go.
FW - Joe Warren. It's true that he's been beaten and gifted a couple of wins but nobody is a better wrestler than this guy at 145. Chad Mendes gets an honorable mention here. I'd love to see these two guys go at it. Chad may make up for the gap in wrestling skill with sheer power, but it's unlikely. Kid Yamamoto gets a 2nd honorable mention here. He was an international level wrestler at 132lbs no less, but like Dan Henderson chose to keep most of his fights standing because of his amazing KO power. Note: I include Yamamoto here because he fought the majority of his fights here and at LW.
BW - Cruz as it relates to MMA. In a straight wrestling match I'd favor Faber. But since we're talking MMA, Cruz is the best.
FLYWT - Demetrious Johnson is the man here and he proved it by using better defensive wrestling than his rival and the 2nd best wrestler in this division, Joseph Benevidez.