General intelligence:
* Sharp as a tack: This implies quick wit and mental agility.
* Bright as a button: This suggests a lively and intelligent mind.
* A whiz kid: This phrase is used for someone young and exceptionally talented or intelligent.
* A quick study: This implies someone who learns quickly and easily.
* A genius: This is a very strong term for someone with exceptional intellectual ability.
Other expressions:
* Have a head on their shoulders: This suggests someone with good judgment and common sense.
* Think outside the box: This describes someone who is creative and original in their thinking.
* Know their stuff: This indicates someone who is knowledgeable and competent in their field.
* A master of their craft: This applies to someone who excels in a specific skill or profession.
The best idiom to use depends on the specific context and the specific kind of intelligence you want to emphasize.