Blackbeard is the obvious choice for Luffy's rival, however I feel like there's someone everyone's overlooking, Eustass Kid. If you think about it, Luffy and Kid share a few interesting parallels with each other, both have a crewmember that are also part of the Worst Generation (Zoro and Killer respectively), Kid's more violent then Luffy and has no problem causing civilian casualties which is why, despite Luffy's confrontations with the Marines, the Seven Warlords and the World Government, Kid has a higher bounty than Luffy at the start of the Sabaody arc. Kid's also willing to kill anyone who laughs at his dream to become Pirate King while Luffy when he was being mocked by Bellamy walked away, claiming it wasn't a battle worth fighting. And then there's each other's relations with Shanks, were by, Luffy looks up to Shanks and Shanks even sacrificed his left arm to save Luffy were as Kid doesn't respect him and as we recently found out, Kid lost his left arm during a fight with the Red Hair Pirates, although whether Shanks took Kid's arm himself is still unknown.
Plus, I remember hearing something about Oda's views on the Worst Generation and that he always intended for Kid to be very important to the plot so he could potentially be a rival for Luffy in the race to become Pirate King, but I guess we'll learn more as the Wano arc goes on.
