Dustin Poirier has the most to lose IMO.
Unlike Gaethje, he has not defended his extremely high ranking against an up-and-comer. He's already talking openly about approaching retirement (that is not to be confused with announcing his retirement in any way lol), which makes you wonder how good he'll be tonight. Think Cejudo: hasn't retired, but totally phoned in his performance against Merab, looked like he'd trained for a round at most.
If Dustin loses, he not only drops out of the top 5, but any hopes of a title shot at any weight class will evaporate.
Likewise, I can't see Marlon Vera getting another title shot ever. This is the big one for him, he simply cannot afford to lose. He's kinda lucky to get the shot considering he's ranked #6 or whatever.
Also, I doubt it would happen, but Gamrot cannot afford to lose to RDA...