What is 2nd most important mission in skyrim?

I just beat the main quest so whats the 2nd most important?

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Answer:
I suppose the civil war quests are also fairly important to the story of Skyrim (Stormcloaks vs. Empire); if you haven't done that, you should pick a side and join up to counter the other side.

After that, everything else is just up to you, whether you want to do it or not. The Dark Brotherhood quests are fun though, in my opinion.

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