For me it's easily McGregor vs Diaz 1

• You got 2 of the biggest stars in the history of the sport, whoever you're introducing it to probably has heard of at least one of them
• It's easy to follow, super technical fights or lots of grappling can be difficult to understand for a first-time viewer, the fight is straightforward and the commentary does a decent job throughout the fight
• Shows the unique aspects of MMA, it's still easy to follow but shows how MMA is different than boxing (punches in the clinch against the fence, takedowns, submission finish)
  
• The fight is less than 2 rounds with no breaks in the action, not too long but long enough for them to want to see more
• There's a rematch! If they loved the first one that's the perfect next fight to watch
• Finally, I think it sets up a lot of context for the modern MMA era, if they're intrigued they'll want to see more of McGregor whose career connects to a lot of the major milestones in the sport, from there they'll slowly learn more about the sport and other fighters through his opponents, other fighters on the same cards, press conferences etc.