I went with Se7en because it was the one that affected me most. I was a relative virgin to that kind of psychologically bent film when I watched it. All of the kills leading up to it are revolting in their own ways and the (second) last one was no different. I couldn't believe a film could be so heartless. After watching it I felt like I had ascended (descended?) to some greater level of horror fan.
Funny, I was trying to do a Brad Pitt impersonation this past weekend and the quote I chose was, "No Jon, we would have caught you eventually."