It can be 1 sec of 1 fight or whatever you want it to be. You don't need to give an answer you think everyone wants to hear, remember this is YOUR greatest moment since you've been watching MMA. I'm at work right now watching The 100 Greatest Fights in UFC history and it came across this fight and it involves my greatest moment in MMA history.
The moment lasts all of 25 seconds but for me, it has to be when Randy Couture dropped Tim Sylvia with that overhand right and rushed over and slammed him, then took his back. That sequence was just amazing. You couldn't hear Joe Rogan speak the crowd was so loud. I've never heard a crowd like that since. So yeah, might not be a popular pick, but for me that's gotta be it.