The reason behind this is simple- I feel that if you haven't watched this movie, you haven't watched anything! This movie literally defines the term 'psychological thriller'! It has a more than decent rating in IMdB and quite a few good reviews.
(Warning- contains spoilers)
Firstly, this movie makes the viewer think from the beginning! In addition to that, Gerard Bulter is the perfect guy for the role of the guy who kills behind bars(that's right; he, in fact, announces each murder before it happens; but then again the supposedly 'good' guys are given chances to avoid them). He brings the character to life. One of his very memorable lines is- "I'm just a law abiding citizen."
The movie starts with the murders of Clyde's(Gerard Butler) family, which is basically a theft gone wrong. The injustice is done when one of them practically walks free after making a deal with the DA while the other; who was not involved in any of the killings directly, gets the capital punishment. The movie then shifts to 10 years later in time, when the execution is to take place, and the mind boggling events unfold one by one!
The best part of the movie, according to me, is that the director has the guts, firstly to kill a few important characters, and then, to have a climax which leaves the viewer guessing whether it's a happy ending or a sad one! More importantly, it raises one important question, "Does the present law system do enough?" Clyde plans his murders in such a way that he remains 'not guilty' to the law! There is a point in the movie when Clyde (who was being convicted of a murder) was about to be let off on bail due to lack of evidence! At that point, he delivers a speech which is very touching and attacks the present justice system(basically, it's the system of the US, but the Indian one is quite similar)! One more satiric fact is that a few of these could have been avoided had the authorities been punctual.
In addition to all these, the climax is really unpredictable(I won't tell you, go and watch it), which is why I feel, it's one of the best (or maybe the best) movie I have ever seen!