Step 1: Understand the word.
Agony means extreme physical or mental suffering.
Step 2: Analyze the options.
Pleasure is a general opposite but not very strong.
Bliss means perfect happiness, close but less intense than ecstasy.
Ecstasy means overwhelming happiness or joy, which is the strongest opposite of agony.
Fear is unrelated.
Step 3: Conclusion.
Therefore, the correct answer is Ecstasy.