Psychological card forcing is by far one of the most strongest effects that I perform. Although not 100% fool proof, when it does hit, it his HARD! My most enjoyed reactions were from this trick alone.
Some versions which I enjoy are
-Derren Brown's mental card forces
-ANATE by Dee Christopher (However I haven't had much luck, must be due to my presentation, which needs refining)
-Psychological Subtleties have great ideas which all serious mentalists/magicians should read.