* Умный (umny): This is the most common and general word for "smart" in the sense of intellectual ability, cleverness, or having a quick mind.
* Разумный (razumny): This word emphasizes rationality, logical thinking, and the ability to make sound judgments.
* Сообразительный (sobrazitelny): This word refers to someone who is quick-witted, resourceful, and able to understand things quickly.
* Остроумный (ostroumny): This word describes someone who is witty, clever, and has a sharp sense of humor.
* Проницательный (pronitsatelny): This word emphasizes having insight, understanding, and the ability to see through things.
The best word to use depends on the specific situation and the nuance you want to convey.