So Oda has established a lot of worthy and formidable foes for the Straw hats to take down. But only one can be the final battle. Given, we haven't met everyone yet, and knowing Oda, there can still be more big players. At the moment a few large ones come to mind. Vote on who is the Big Bad.
Below are info on the possibilities, spoilers of course:
! Blackbeard: He seems most likely currently, being the pirate crew that most directly opposes the Straw Hats. He and Luffy already hate each other, and have the same goal of finding One Piece. Most signs point to Blackbeard being the ultimate villain of One Piece. Now that he has risen to the position of Yonkou and has a insanely strong crew, the Straw Hats are going to have to be ready before challenging them.
! Akainu: Now that he has become Fleet Admiral, Akainu has all the authority to pursue the Straw Hats across the globe. He is powerful enough to have defeated Aokiji, and Luffy has a grudge against him for killing Ace. He is truly a perfect enemy for Luffy. However, not being a pirate, he is more a constant obstacle rather than a final villain. But who knows
! Doflamingo: Clearly Doflamingo is a big player in the world of One Piece. He seems to have a hand in every organization and major event in the series. The long suspected mastermind of everything corrupt, the "Joker" has directly effected Luffy and the crew with his Auction House, Bellemy's Crew, and most recently on Punk Hazard. Luffy definitely has beef with this guy, but as a final villain, Doflamingo seems to fall short. Sure his morals and ideals are great for a final villain, but he is more of a final villain of a Saga, much like Crocodile and Lucci, except far more important in the long run. That being said, I personally believe this current saga will revolve around and end with Doflamingo, but we'll see
! Big Mom: Now we are getting to the far less likely. Big Mom and Luffy are already planning to fight, however, when will be unknown. If Big Mom is indeed the Yonkou that Law plans on taking down, then she will probably be taken out in the near future. But being a Yonkou, she is powerful enough to be a final villain. But too little is known.
! Smoker: One of Luffy's biggest rival, Smoker has yet to stop his pursuit of the Straw Hats. Although he is a Vice Admiral now, I don't think he really is Big Bad material. In fact, considering his personality, I'd be willing to bet that Smoker will be at the final fight, but on the side of the Straw Hats. At the least, Smoker definitely won't be taken out til the very end of the series.
! Buggy: Luffy's other reoccurring rival. Buggy is definitely not final villain material, however, like Smoker, I think its worth noting his importance, and once again, I believe he'll be there at the end
! Other characters along the lines of the last two are Crocodile, Aokiji, and Mihawk. It's not that these characters won't be final villains, but it just seems far more likely that in the end, they will be at the final fight, but on the Straw Hat's side along with all their other friends like Mr. 2, Law, Jimbe, Hancock, Shanks, the Revolutionaries, the remaining Whitebeard crew, and so on….
! Two other noteworthy characters are Kaidou and Kid, but too little is known about either of them, that it doesn't seem right to include them.