Individually dangerous, I think Skyrim's dragons surpass anything Valenwood has.
Obviously, you and I have had very different experiences in skyrim
But on a serious note, it's not as though Dragons are indeginous to Skyrim, or even exist there under normal circumstances. The fight between Wild Hunt and Dragons would be interesting, and eventually won by the Dragon, but before the time of TESV, there were no dragons in Skrim, and after TESV, dragons are no longer exclusive to Skryim. Wild Hunt, Bull Netch, Poison Swamp Beasts, intelligent horse-sized tigers, and the things depicted in the tapestries that once hung from Crystal-Like-Law are all far more threatening than Mammoths, imho.
However, if we qualify Giants as monsters, I suppose Skryim definitely deserves a spot in the top 3. However, I'd personally classify them as non-threatening aboriginal peoples.
I don't see the wild hunt being that deadly if the Aldmeri Dominion managed to take Valenwood *shrug*.
The AD took Valenwood mostly through politics. Also, you don't need to fight every living thing in a province in order to have control of it. The Thalmor probably never fought the WH.