I'd go Dawnguard, those random vampire attacks can be rather annoying... especially when they kill non-essential npc's you might need, such as merchants
The random vampire attacks are a huge nuisance but it's not too difficult to help prevent civilians from dying. The attacks mostly happen upon fast traveling into cities and when you see a civilian drop, reload the autosave then fast travel back to the city. This time quickly get their attention on you first. I've never lost a civilian yet and I've never had to reload more than once. Big nuisance, but avoidable.
Dawnguard. Even though Dragonborn is better. Save the best for last though. Otherwise, you're going to get bored of Dawnguard for not being like Dragonborn.
That's kind of what I was thinking too. Save the best for last.
They're both really really great DLCs though.