I think the Saw series gets my vote, with #6 being the best one. - A tapestry of twists and turns that will keep you guessing the entire series, great individual deadly scenarios that make the audience question what they would do in the same situation, sends a message to appreciate your life and not to take things for granted, and the bad guy SURVIVES! lol The Saw series is seriously outstanding, I definitely recommend it!
Honorable Mention
-Terminator2 - great plot, amazing action, realistic effects, exceptional acting from Robert Patrick as the T1000, overall awesome movie, definitely one of my favorites
-The Matrix - EPIC movie! Outstanding special effects, amazing plot, decent acting all around with Laurence Fishburne and Hugo Weaving both at their best, and subtle metaphors and allusions throughout the movie, also created an entire new form of action sequence - which has been imitated dozens of times since, but never perfected like the Matrix did it
-Starship Troopers - Great plot with well developed characters, a unique movie that stands alone in the sci fi genre, surprisingly great special effects and believable dramatic performances, also shows influences of a big brother type society with emphasis on the military
-Shawshank Redemption - great movie, outstanding plot and award winning acting from Tim Robbins and Morgen Freeman, great ending