In order it's:
1) Conor vs Khabib 2, and it isn't even close. The first one was massive, the camps have real beef, build up would be crazy, Conor's chance for redemption, high stakes.
2) Conor vs GSP. Potentially massive, but it's just never gonna happen.
3) Conor vs Masvidal. Both are good promoters. Masvidal is a decent draw, even by himself, his Askren KO and BMF belt put him all over ESPN. Together with Conor, if they build it right, it would do huge numbers, Mas is a real threat.
4) Jon vs Brock. Would be massive too, has been talked about a bunch. But probably impossible with USADA in the mix.
5) Honorable mention: Khabib vs GSP, freakshow boxing fights like Conor vs Floyd/Pacquiao or Khabib vs Floyd. None are ever gonna happen either.