I love Tim Robbins and Morgan Freeman, so my opinion may be a little skewed for Shawshank. But, I read the book and thought, unlike other movie adaptations, at least did it a little justice (pun only slightly intended).
The simplicity of a character like Andy is what makes him so complex. His, unpredictability keeps you thinking about what he is doing, what he is up to, why he is the way he is, and why it sucks so much for him to be in prison. His intelligence and kindness, even in prison, makes him so likable and watchable. Plus, all the other characters have their own issues, their own truths, their own personalities. I could watch this film 100 times over and still find little things here and there that I missed before.
It is just one of my favorite movies, ever, for a bunch of reason. Might not be your favorite, but I don't think I have ever heard anyone call it a bad movie.