What is the word opposite of brilliant?

The opposite of "brilliant" depends on the context. Here are some options:

For intelligence:

* Stupid

* Dumb

* Dull

* Unintelligent

* Ignorant

* Uninformed

For light:

* Dim

* Dark

* Faint

* Cloudy

* Opaque

For color:

* Pale

* Dull

* Subdued

* Muddy

For success:

* Failed

* Unsuccessful

* Ineffective

* Mediocre

For creativity:

* Uninspired

* Boring

* Unoriginal

* Bland

To choose the best opposite, consider the specific meaning of "brilliant" in the context you are using it.