I believe that Blackbeard is not the strongest Yonko even after he gets WB fruits. So people who can beat Blackbeard are
1) Luffy. Reason - Luffy if become Gear 4th and does King Kong gun then I think BB wouldn't be able to handle it.
2) Marco. Reason- I put Marco in it because I think Marco didn't retreat because of he losing against him but his people getting Injured. Marco can defeat him because of his regenration ability. Even if BB attacks in his full power then also he will regenerate.
3) Akainu. Reason - BB is scared of Akainu and he believes that he will get destroyed so maybe he will be stronger but due to thinking negative he will lose
4) Red Mihawk. Reason - If Mihawk does his slash then it might cut him in half. He can't deflect it or break it but he might absorb it but he will have to feel the pain of getting the body cut in two. If Mihawk uses Haki then BB might die.
5) Shanks - Reason - He is a Yonko but no one knows his true power but by judging his conflicts with WB, he might be able to beat him.
6) Dragon - Reason - the world worst criminal, Dragon can beat him though we may not know much about him but he may control the climate, proofs can be seen in loguetown.
7) Kaido. Reason - Kaido being a giant he can beat BB as he can mot be killed and we are not sure that he has devil fruit. Kaido is also a Yonko. What more should I say to prove this fact?
8) Big Mom. Reason - I believe that Big Mom is the weakest Yonko and is only called Yonko because of her sons and daughters. Her soul taking won't work on people who have strong Haki. As we are not sure that Blackbeard has stronger Haki then Big Mom therefore Big Mom can just suck his soul.
9) Rayleigh. Reason - He was the vice captain in Gold D rogers crew. He has Haki better than anyone in One piece. So using Haki and his own strength, this awesome guy can beat him.
If there are more than please comment.
If you don't believe what I think is right then please comment on where I was wrong.