General intelligence:
* Intelligent
* Smart
* Bright
* Sharp
* Astute
* Brilliant
* Gifted
* Talented
Quick wit and ingenuity:
* Clever
* Ingenious
* Resourceful
* Shrewd
* Wily
* Cunning
Deep thinking and understanding:
* Wise
* Insightful
* Perceptive
* Knowledgeable
* Learned
* Erudite
Other options:
* Savvy
* Quick-witted
* Sharp-minded
* Keen
* Discerning
* Profound
* Sagacious
The best word to use will depend on the context and the specific qualities of the person you are describing.