The suitable modal to complete the sentence is: used to.

The complete sentence is: "I used to drink tea, but now I prefer coffee."

Here's why:

• "Used to" indicates a past habit or action, which perfectly fits the context of the sentence. The speaker is mentioning a past action of drinking tea and comparing it to their current preference for coffee.
• "Ought to" expresses obligation or strong recommendation, which doesn't fit the context. The sentence isn't about suggesting or recommending anything related to drinking tea.
• "Need to" implies necessity or requirement, which doesn't apply to the speaker's past habit of drinking tea.
• "Dare to" suggests a challenge or risk involved, which is irrelevant in this context.

Therefore, "used to" accurately conveys the speaker's past practice of drinking tea before their preference shifted to coffee.