The strongest possible effect in mentalism is the Question and Answer act. However, since you are asking in regards to mental magic, (close-up and stage), I will assume that you are refering to a one on one effect (where you reveal one thought), which is still a very strong effect.
I would highly suggest that you read the book "Peek Performances" by Richard Busch which will give you more than enough techniques to learn how to glean secret information from your audience member (for both close-up and on stage) to perform design duplications (a very powerful effect) or to reveal secret thoughts of that person.
Another very powerful technique is found in Ted Lesley's Paramiracles in his Disertation on the Centre Tear. His technique for revealing the thought of person's name, who they are and phone number is worth double the price of the book.
Hope that helps,
Paul Alberstat http://www.bigfoot.com/~thoughtreader