I'm watching Lucha Underground season 1 on Netflix and talking to Matrix over on Discord and during season 1 I have seen former WWE stars like John Morrison/Johnny Mundo, Davari, Ezekiel Jackson/Big Ryck as well as Alberto El Patron/Del Rio & Chavo Guerrero all tear the house down in ways they really weren't allowed to in the WWE. all of these guys have had some measure of success after WWE but ho had the most? you don't have to pick from my small list and they didn't have to have performed in LU either but out of all the ones I've been able to see I have to pick Johnny Mundo. The man kills it in LU as either a face or a heel and is a triple crown champion in the company. Through LU's working Relationship with AAA and became a huge star in that company going all the way to the top when he beat El Texano Jr, El Hijo del Fantasma to win the AAA Mega Championship & AAA World Cruiserweight Championship as well as retain his AAA Latin American Championship (That he won from Pentagon JR )all in the same Triple Threat match. He still holds all 3 titles as of me typing this out. I can't think of anyone that has made it bigger after leaving WWE than that.