What's a word for the exact opposite of regret?

Solution 1:

In the most general sense, the opposite of regretting something is to be proud of it.

To extend this further, the opposite of your biggest regret is your proudest moment.

Solution 2:

I would say the opposite of 'biggest regret' is 'greatest satisfaction'.

Regret is what is felt if one is dissatisfied with an outcome.

Satisfaction is the feeling of achievement, of success.

2a: fulfillment of a need or want b: the quality or state of being satisfied : CONTENTMENT c: a source or means of enjoyment : GRATIFICATION

Merriam Webster

As noted in comment, the wording 'greatest satisfaction' accommodates to the OP's idiomatic wording 'biggest regret'. 'Big' is a matter of bulk, of burden.

'Great' is a matter of expansiveness, fulness, release.

This is the greatest satisfaction of my life - Shakira, UNICEF Ambassador.

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