I know it might seem odd, but the two most notable ones for me are Jones and Conor. They were both their own worst enemy. I get that one is considered a GOAT and the other the most financially successful one, but they are both absolute trainwrecks.
Had they approached their careers more like GSP had, without the Dana haterade of course, , their bank accounts would be 2-3 times what they are now and their records would be markedly improved. The long layoffs, and character fuck ups really held them back. They both would have had the clout to really force the UFC's hands if they wanted and be a real advocate for fairness for the fighters.
Had Nick Diaz taken his fighting career more seriously, he could have been a fan favourite for years and years. Not on the same level as Jones and Conor as far as squandered talent goes, but up there.
Shane Carwin also comes to mind.