First thing's first I'm going to set a bar at 25 fights I think some guys like Johny Hendricks(17 fights), Jon Jones(20 fights), Shane Carwin (14 fights) and Chris Weidman(11 fights) it's a little early in their careers to call them the best ever. I'm also going to eliminate women, bantam weights and flyweights because finishes are rare.
(HW) Bas Rutten(3) - While I consider him to not be a true HW he was a HW champion and frankly if it's not him you have to look at Heath Herring and he had four TKO's that were "technical"
(LHW) Gegard Mousasi(7) - 40 fights, kinda a default pick as I can't come up with anyone else at 205 who did as well.
(MW) Hector Lombard(6) - 39 fights not a single KO, and in most cases I think guys refused to fight and stand with him.
(WW) Carlos Condit(2) - 36 fights not a single KO, and I can't recall ever seeing him dropped once.
(LW) Takanori Gomi(4) - 44 fights number of loses but one of the ten greatest LW's of all time
(FW) Jose Aldo(1) - 25 Fights right at the cutoff
The rest of the top ten in my eyes.
HM - Diego Sanchez(5) - 30 Fights going head first and really just a doctor stoppage personally I feel like he would have pealed off his face and kept going.
HM - Jim Miller(9) (Sanchez fought for the title Miller didn't, Sanchez a much higher level of strikers)
HM - Chris Lytle(10) (only doctor stoppages for Lytle which considering how long he was around is impressive)
HM - Gilbert Melendez(8) (should likely be ranked ahead of Gomi and Sanchez but I just don't think he's been in danger of getting KO'd in his 25 fights)