Royce Gracie is an obvious legend in MMA. I chose to put two of the members on the Gracie clan as honorable mention because neither one quite fits the mold as, "former fighter who is still relevant in MMA." Keep in mind I tried to make sure these were all "ex-fighters" and tried to justify their current contributions.
Please feel free to add other fighters below that you think should have made this list. Here is a list of fighters that are still relevant and will likely continue to be relevant in the sport long after their fighting days are done. However, there are some fighters that continue to stay relevant in the sport and keep contributing to the sport even though they no longer step inside the cage themselves to fight competitively. Over the past 19 years, most fighters that created legacies in the ring/octagon became disconnected from the the sport after they retired. While Royce Gracie was displaying his dominant fighting skills on the ground, dominant strikers like Bas Rutten were displaying dominant standup skills and evolving that aspect of the sport. Since Royce Gracie introduced most of the world to Jiu-Jitsu and won three of the first four UFC tournaments without suffering a defeat, the sport has seen its share of dominant fighters come and go to help mold the sport as we know it today. The sport of mixed martial arts as we know it is approaching 20 years of age.