If you're talking about similarity:
* Different: This is the most general opposite, meaning not the same.
* Unlike: This emphasizes the lack of similarity.
* Dissimilar: This is a more formal way of saying unlike.
If you're talking about two things being equal:
* Unequal: This means not having the same value or quantity.
* Disparate: This means completely different or distinct.
If you're talking about being similar in appearance or nature:
* Distinct: This means clearly different and easily recognizable.
* Unique: This means one of a kind and not similar to anything else.
The best opposite for "alike" depends on the specific situation and what you want to emphasize.