Blackbeard at this point I would have to say is definitely rising to the top. I wouldn't say he's the most powerful man in the world, but he has a lot under his belt to prove his strength.
Let's start with the fact that he was one of the Whitebeard pirates for decades. He was at the earliest shown in Chapter 0, when Shiki came aboard the ship speaking of his plan to Whitebeard. Which means he's had two decades to learn from Whitebeard and train himself physically. If he was planning things from that far back, it would only make sense that he was working out most of the other parts of his plan. 20 years is a lot of time to formulate a plan, especially with the freedom granted by being a Whitebeard Pirate. He was nominated to be the 2nd Division Commander, which he gave to Ace to stay as low as possible. This already is evidence of his battle prowess. During this time he had fought with a Yonkou, Shanks, and was able to give him 3 deep scars; not just physically, but mentally. Then at the end of his run as a member of the Whitebeard Pirates was able to battle with Thatch, Fourth Division Commander of the Whitebeard Pirates and kill him with no lasting scars of his own. (Note: This was before he had the power of the Devil Fruit mind you)
After he had left, Whitebeard himself was against having people go after Teach because he felt very uneasy about the situation. When Whitebeard himself doesn't feel strongly about something to the point of worry that is a terrible omen. Two Yonkou fear his intentions and the outcome of what he's going to do, if that isn't enough of a sign I don't know what is. He was able to demolish all of Drum Kingdom to the point of causing the Governing forces to leave entirely. He was also looking for a way to get into the Shichibukai, which meant he was confident in his abilities enough to hunt high profile criminals and subdue them.
Most notably is the application of his devil fruit and his defeat of Ace, which makes two Commanders of the Strongest Man in the World's fleet he defeated. Ace was a very big pirate across the seas and fought with many of the series strongest characters, causing a very big commotion throughout the World Government. He was also going to be one of the Shichibukai at a very young age, probably the youngest in the series, shortly after setting out to sea. Defeating him was nothing short of incredible in terms of strength.
The way he is now with both the powers of the Yami Yami no Mi and Gura Gura no Mi, his power is absolutely nothing to laugh over. Is he the strongest person in the series? I wouldn't necessarily say that, but he's up there. With time he'll definitely be one of them, which has been stated at least once during the series.
