You don't really have that many options for dragon types. You can only choose Flygon, Haxorus, Druddigon, Altaria, Hydreigon and the boxart legendaries. If your playing bw, then you can cut off Flygon and Altaria off that list
Naturally, the boxart legendaries are going to be the best options since they're offensive stats are significantly higher, most of the time anyway but the problem is that they come very late in the game in bw and I don't even think their available before the champion in bw2
So, with that in mind, the best option is Haxorus for both games. Why? Dragon dance and mold breaker. Gen five is infested with the ever irritating sturdy so ignoring that completely is a huge blessing. Also, dragon dancing is insanely powerful in playthrus. Just find a mon that can't threaten the dancer, click dragon dance two or three times and you one shot the entire enemy team. Just give Haxorus brick break for its coverage option since you can't get equake until post game. In terms of availability, it is available mid game at mistralton cave so it isn't that late, unlike a certain Hydra we all know and love
In bw2, Flygon can give Haxorus a good run for its money as a dragon type. It's stats is a bit meh but it does have an incredible stab combination, levitate and learns rock slide by level up, which is a good coverage move. Perhaps most importantly, it is available in the desert right before the electric gym. Flygon also has the advantage that its not as difficult to level up as Haxorus. This thing however, is not available in bw
Those two are really the only good ones. I won't recommend Hydreigon because not only does this thing become available so late, it is a massive pain in the ass to level up. In fact, its a pesudo legendary, which means it has a very slow xp growth and it evolves at level sixty four. Unless you're really that patient, just don't use this. It's colorful movepool just isn't worth it when compared to Haxorus. This applies to both games
I also don't think that Druddigon and Altaria should be used. Altaria is complete garbage and Druddigon is too slow. Drudd does have a bunch of cool options like night slash and superpower but with how slow it is, you'll probably just kill one enemy mon and die to the next one. As such, the verdict for both games is Haxorus
