For Dragon, as the World's most wanted man, he is definitely strong. However, exactly how strong can't be concluded at this time.
If you compare Dragon to a Yonkou, you could say that he was either strong or weaker or the same strength.
- Stronger: The WG sees all pirates as a threat, especially the Yonkou, which is why the Marines and Shichibukai are there to balance off the Yonkou and their crews. However, they see Dragon as more of a threat, therefore they go after Dragon, which could mean that he's so strong that they can't just leave him alone.
- Weaker: The WG never intentionally picked a fight with the Yonkou before now, since they were afraid of the repercussions, which is why they made the marines and shichibukai as a guard against the former. However, they saw Dragon as, albeit a bigger threat to the world's stability, physically weaker. Therefore, they have been after Dragon from the get-go, since he's weaker and easier to face.
- Just as Strong: When Ivankov found out Dragon was Luffy's father, and incorrectly came to the conclusion that Dragon was also Ace's father, he was dumbstruck as to why the WG would face WB AND Dragon. By putting them in the same sentence, thus equating the latter with the former, Oda could have been freshadowing Dragon's strength. After all, WB has about 2000 pirates (or w/e the number was) under him, which thus speaks volumes about his own strength. Similarly, Dragon having the entire Revolutionary army (a group of strong people who openly oppose the WG and embrace certain death) under him also speaks volumes of his strength.
As for who is the strongest now is up to speculation. We don't really know how strong Shanks, Kaidou, and Lola's Mom/sister are personally, as well how strong Dragon and Blackbeard are as well.
- Shanks playfully fought WB, and spoke to him as an equal, therefore we know he is definitely strong. We don't know how big his crew is though, or how strong they are. We do know BB left a mark on his face, and he was equals with Mihawk 10 years prior. However, Shanks true strength is left to speculation for now.
- Kaidou was willing to face WB at full strength in order to prevent the latter from going to Marineford. This speaks volumes about himself, and leaves us to wonder if he saw WB as an equal. No one else would go to fight WB for shits and giggles. However perhaps he knew BB's plot, or how strong BB was. His reason remains unknown until Oda tells us.
- Lola's mom/sister, well we donno shit about her. She could be the strongest though.
- Blackbeard was strong enough to injure Shanks, but got rolled by Megallan and WB. Now that he has WB's power he's definitely stronger. However, having the DF power means jack shit. It's not about having the power, but rather how you use it. We've seen before that BB just stands around and doesn't really know how to fight. So, yeah. Personally, I don't think he's the strongest. Roger was able to face WB head on (during WB's peak) against the quake fruit. Also, haki can probably pull a number on the yami fruit. that and attacks can go through the yami fruit. The only people that DF's power has worked on was Ace and a bunch of fodder marines. And as we all know, Ace wasn't as strong as some others in the OP world.
- Dragon can manipulate weather, runs the biggest army next to the marines (which got slaughtered btw), was able to subdue Smoker (then again Smoker's weak shit. He's not an admiral or a vice, the only reason he beat Luffy was because of the seastone stick, and Boa whooped his ass pretty badly), and is a generally bad ass mother fucker. However, we don't know how strong he is in relation to others, so, meh. He's definitely strong, but the title of being "The Most Wanted" doesn't mean much for actual strength.
So yeah, there's no point in discussing how strong people are, because really, we don't have sufficient proof for the matter. Black Beard says he's the strongest, but then again, he's a cocky mother fucker. If Shinryu didn't save him, he would've died to Megallan; and Megallan was definitely not the strongest.
Imo, Rayleigh is the strongest. He was Roger's right hand man, and WB didn't have a right hand man. it also looked like Rayleigh was playing around with Kuma and the Admirals, since he has no real reason to fight people with full force. After Rayleigh would be Sankou/Dragon, then Black Beard.