1. I'm down.

= I don't feel well.

2. I'm _____
= I feel good.
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What is the suitable word for the blank?

The opposite of "down" is "up."

To find the suitable word for the blank, we need to consider the opposite of not feeling well or being down. A word that conveys feeling good would be appropriate for the blank.

2. I'm feeling great.
= I feel good.

Therefore, the suitable word for the blank is "feeling great."