Honestly, Akainu seems more like the final boss to me. Blackbeard is the opposite of Luffy yes, but he's not totally evil as evidenced by our first meeting with him. I think the two of them will fight on Raftel over the One Piece, set off the big war and then Akainu will be the big bad. He's currently in charge of the marines and we know him to be not only cold and ruthless when it comes to carrying out his own brand of justice but also that he will ignore orders in order to carry out his justice. If he thinks Luffy has the One Piece and it threatens the world then no one will be able to stop him from doing whatever it takes to kill him. He's very much a loose cannon and perhaps the most dangerous person in the manga right now because of it.
Sakazuki would make for an extremely disappointing final villain. I pose the question again: What kind of emotional investment would Luffy or the readers even have in such a fight? To be fair, Sakazuki did kill Ace. But getting revenge for Ace's death is hardly the driving goal in the series. But finding One Piece? Much like Blackbeard is trying to do? Which puts him in direct opposition to Luffy's drive and dream? Now that sure is.
Not to mention, the lore and history of the entire series are rooted in the void century. Blackbeard, as a D., has direct connections to that history. We don't know the significance of the ancient kingdom yet but it
is significant, to be sure. And Blackbeard will tie into it.
Sakazuki only cares about Luffy because Luffy is a pirate. That's literally his entire ambition for wanting Luffy dead. What kind of final villain would that be? One who only opposes the main character by default because they are on enemy factions? Sakazuki has no larger goals or ambitions other than wiping out pirates. He is no more than an attack dog, killing pirates for the simple reason that they are pirates. He is ruthless, but as a character he is shallow. Hardly fitting for the climax of a series.
Blackbeard, on the other hand? Whose ambitions directly clash with Luffy's? Who has a larger goal in mind? Who has connections to the void century? Who has been scheming his rise to power for several in-story decades now? Who both literally and figuratively usurped the position previously held by the world's strongest man? Who has been portrayed as an evil version of Luffy? Whose ideologies on what it means to be pirate king are the exact opposite of Luffy's (freedom vs. tyranny)?
Akainu, in my mind, will be the penultimate villain. His fight with Luffy will be the culmination of the war between absolute justice and true justice. That, though, is a discussion for another time. But the fight against Blackbeard - that will be the clash of ideologies and dreams. A culmination of the entire series' themes.
There are multiple layers to the conflict between Blackbeard and Luffy. Not so with Sakazuki.
EDIT:
Also, Sakazuki is hardly a loose cannon as you say. When he really gets into it he does indeed go overboard, but as we saw in Dressrosa, he still takes commands from the government. Talking about the scene on Green Bit here.
