Shanks is, at the very least, close to a match with Whitebeard. Dragon, powerful as he is, likely cannot match that level.

But the world government considers Dragon as the most dangerous person alive.. If he is a pirate, I am sure he will be a yonkou too..

"Most dangerous" is not the same as "most powerful." There's a reason Whitebeard held the latter title and Dragon the former - he presents a threat that goes beyond pure strength, organizing an army that constantly challenges the authority of the World Government. Is he powerful? Certainly. Can we assume that his power is comparable to that of Whitebeard? Possibly, though we have little to no basis for doing so. Meanwhile, we have a very solid basis for assuming that Shanks was, at the very least, close to a match with Whitebeard. If we're going to err here, we should err on the side of the knowledge we have rather than assuming based on titles.

I agree with Flame. Dragon's title is vastly different to Shanks'. Shanks is more than likely a match for Dragon, since he wields a strong mastery over each of the Haki and is one of the most powerful swordsmen in the world, as well as being a Yonkou. Dragon's abilities are mostly unknown - we know he has some kind of control over the weather, as we saw when he saved Luffy from Smoker, but we don't know if he has Haki, though we could assume that he does. I'm all for Shanks on this one, as we know exactly how powerful he is.