【Intelligent】 ,  【clever】 ,  【alert】 ,  【quick-witted】 ,  【bright】 ,  【smart】 ,  【knowing】 ,  【brilliant】  are comparable when they mean mentally quick or keen.

【Intelligent】 implies greater than average power to use one′s mind successfully when demands are made upon it (as in understanding the new or abstruse or in meeting and solving problems).

【Clever】  implies resourcefulness or aptness more strongly than  【intelligent】 , and it emphasizes quickness in apprehension rather than fullness of comprehension, and dexterity or adroitness, rather than soundness, in the mental processes. Often the word suggests a contrast with higher or more substantial qualities.

【Alert】 stresses quickness in the mental processes, especially in comprehending a situation.

【Quick-witted】 also implies quickness in thinking but in addition it suggests promptness in action in an emergency, in response to a challenge, in conversation, or in debate. 【Bright】  and  【smart】 are more often applied to young or promising persons than to those who are proficient or of proved intelligence.

【Bright】 suggests cleverness that is manifested especially in liveness of mind or in liveliness of talk or manner.

【Smart】 , too, implies cleverness but it also suggests alertness or quick-wittedness that enables one to get ahead. Both words are used ironically,  【bright】  then implying dullness or stupidity and  【smart】 , pertness, facetiousness, or sometimes trickery or duplicity.

【Knowing】 carries a stronger implication than any of the preceding terms of the possession of information or knowledge that is necessary or useful under given circumstances. Occasionally the term further suggests a less agreeable quality such as sophistication, secretiveness, or the possession of knowledge of others′ secrets.

【Brilliant】  adds to  【intelligent】 the implication of unusual and outstanding keenness of intellect that manifests itself so openly or effectively as to excite admiration; the term usually suggests an opposition to qualities that characterize one whose mind works more slowly or cautiously.