Fill in the blank: You were late for an important meeting. You ______ home earlier.

The correct answer is should have left.

• The sentence describes a past situation ("You were late for an important meeting") and implies regret or a missed opportunity.
• The structure "should have + past participle" is used to express that something would have been a good idea in the past, but it did not happen. In this case, it means you regret not leaving earlier.
• "Must have + past participle" is used to express a strong deduction about a past event, not regret or a recommendation.
• "Should had left" and "must had left" are grammatically incorrect forms.

The completed sentence is:
You were late for an important meeting. You should have left home earlier.

The final answer is $\text{should have left}$.