At the Legendary version I can only say - TOP! I had just lent me Skyrim and was very pleased that I was able to acquire it for the best price together with all extensions.
When I then saw that I could incorporate into my current score the extensions, I was more pleased, think this is not a foregone conclusion.
As for the extensions, I personally have a clear ranking:
"Dragonborn" is the best extension, alone the purchase as a TES-fan already pays for.
A new island with numerous caves and an impressive new main quest series provides voltage to the last breath. There are also new cries, new equipment and clothing, new enemies ... just good!
"Dawnguard" I played personally not big. I think the extension is also very good, but I've just used that to have a second attendant Vampirella Serana and the main quest is therefore not played a relatively early stage. Will I catch up sometime.
"Hearthfire" is difficult to compare with the other two extensions, for me it's more like a fun gimmick. Since the game is offline, while others can not visit your own house that I lack there a little. However, it has still a lot of fun to cobble at home around. One can indeed build up to three and also adopt children, which in itself is a nice idea.
So strong buy recommendation for anyone who does not already own or Skyrim Games of this kind hates)