I can sort of see why some might say Akainu. He's the head of the Marines, a constant force and recurring threat to the Straw Hats and the rest of the pirating world. Not to mention he's pretty much presented himself as justice incarnate(his own brand of what he sees as just, that is), gave Luffy his scarred chest, and of course, the biggest factor of all; the fact that he was the one that punched the hole through Ace. He's a major threat and enemy and someone that will end up trading blows with Luffy someday.
HOWEVER
Ever since his introduction in Jaya, Blackbeard has been presented as a dark mirror of Luffy. A man who believes wholeheartedly in dreams and won't stop to accomplishing them no matter what he's up against. He'll use any means necessary and play as dirty as he needs to fulfill his goals. As far as final enemy, besides the fact that his crew itself seems to be a twisted villainous version of the Straw Hats, Blackbeard killed Whitebeard, bringing about the new pirate era, as well as was responsible for Ace even being under threat of execution. He started off in the series as a leader of a small band of pirates but has grown in strength and infamy, just as Luffy did.
I know everyone's entitled to their own opinions and that people are perfectly free to believe Akainu, the WG, the Gorosei, any of them could be the final antagonists but so far from what we've seen in the story, it seems clear that Blackbeard is being set up to be the big bad.