If "plain" refers to simplicity:
* Elaborate
* Ornate
* Fancy
* Intricate
* Detailed
If "plain" refers to lack of decoration:
* Decorated
* Ornamented
* Embellished
* Adorned
* Dressed up
If "plain" refers to being straightforward:
* Complex
* Complicated
* Intricate
* Vague
* Ambiguous
If "plain" refers to being ordinary:
* Extraordinary
* Exceptional
* Unique
* Remarkable
* Spectacular
If "plain" refers to being simple in taste:
* Sophisticated
* Gourmet
* Exotic
* Delicate
* Flavorful
If "plain" refers to being clear and understandable:
* Obscure
* Vague
* Ambiguous
* Complex
* Confusing
To determine the best antonym, consider the specific context in which "plain" is being used.