The appropriate modal to complete the sentence is: ought to

The complete sentence is: "The party was very entertaining; you ought to have been there."

Here's why:

• "Ought to" expresses obligation or strong recommendation. It suggests that the speaker believes the listener would have enjoyed the party if they had attended.
• "Need to" implies necessity or requirement. It doesn't fit the context as attending the party wasn't mandatory.
• "Dare to" suggests a challenge or risk involved, which is not relevant in this context.
• "Used to" refers to past habits or actions. It doesn't fit the present-tense context of the sentence.

Therefore, "ought to" accurately conveys the speaker's strong belief that the listener would have enjoyed the party if they had been present.