If "better" is used in a comparative sense:
* Worse: This is the most common opposite, meaning "of lower quality" or "less desirable."
* Inferior: This means "lower in rank, quality, or importance."
If "better" is used in a more general sense:
* Worse: This still applies, meaning "less good" or "undesirable."
* Poorer: This implies a decrease in quality or condition.
* Bad: This is a general opposite of "good," which "better" implies.
Other possible opposites:
* Worse off: This implies a negative change in someone's situation.
* Not as good: This is a more neutral way of expressing that something is not better.
The best opposite will depend on the specific situation and the nuance you want to convey.