Both are among the greatest antagonists One Piece has ever produced. I personally prefer Akainu but in terms interaction with the protagonist and actions within the manga, Blackbeard beats him.
What makes Blackbeard more interesting in the Luffy interaction department is the fact that the interaction between these two has not always been negative.
1) Blackbeard was the only one in Mock Town who encouraged Luffy to find Sky Island and to pursue his dreams.
2) Blackbeard and Luffy inadvertently assisted each other during Impel Down. Magellan was only outmaneuvered because these two acted at the same time. If only even one of them had acted at a different time then Magellan would have destroyed both groups.
Meanwhile, Akainu’s only interaction involves fisting the protagonist’s sibling to death. Lol.
Blackbeard’s actions led to the death of the most legendary Yonko, the destruction of the aforementioned Yonko’s entire crew, elevated himself to take the available Yonko seat and started a whole new era.
Akainu became a higher ranked cog in the Marine machine.
I expect both of these fine individuals to achieve great feats in the future.