Depending on the context, some good antonyms could be:
* Calm: This is a general antonym for being flustered, which implies being nervous or agitated.
* Composed: This implies being in control of one's emotions and actions, the opposite of being flustered.
* Collected: Similar to composed, this suggests a state of being calm and focused.
* Relaxed: This conveys a sense of being at ease, the opposite of being flustered and stressed.
* Serene: This implies a state of tranquility and peace, the opposite of being flustered and anxious.
The best antonym will depend on the specific context of how "flustered" is being used.