Its hard to narrow it down year by year, but MMA to the mainstream was basically...
90s: Ken, Royce, Severn, Frye and Tank
00s: Chuck, Randy and Tito for most of it but the later part turned to Brock, GSP and Anderson
10s: Early part was still Brock/GSP/Anderson with Cain entering the fray, and Jones entering the Fray a bit after that. 2015/16 was Conor and Ronda. Since then we are in kind of a weird place where Conor is still the face of MMA to casuals but is extremely inactive, and Ronda is gone too.
(Obviously Fedor was a big part of 2000s MMA to the hardcores, possibly the defining face of it)