2. Black Swan
Not exactly marketed as a movie about mental illness, anyone who's seen it can say it's the story of obsession and stress driving someone to the breaking point. Others focus on the scene with Natalie Portman and Mila Kunis, but that was just to get certain demographics into the seats. The film follows Natalie "Lady Thor" Portman as she prepares for the key role in Swan Lake. As mentioned in pick 1 of this list, we find a parent with their own issues (a former dancer with dreams of living through her child). That, plus competition from Mila "Meg" Kunis leads Natalie down a dark path that ultimately leads to some pretty trippy insanity. Scary, sexy, and ultimately a window into the mind of those obsessed since birth about one thing, and how they react when they get it.
What really ruffles a lot of guys' feathers (heh) is how anyone could do anything to Natalie as long as they add the phrase "it's going to make you a better dancer." Her instructor molests her but then says her reaction is what he was trying to bring out. Mila does...stuff...and the same reasoning is used. To see someone give up everything they are just to get something they want is sad.
Back when Russell Crow was still an actor and not a cage fighter, this was one of his best works. Based off a true story, Russell plays a mathematician who suffers from paranoid schizophrenia and we track his life and career as the symptoms become more severe. The great success of this film is that both Crow and the audience are introduced to the hallucinations so gradually that we don't realize by the end that half the movie didn't even happen. Such is the difficulty that Crow dealt with, trying to react to people who weren't there while alienating those that were. The one unrealistic part is that in the end, Crow uses his intellect to "out-think" the hallucinations and ignore them without medication. Sadly that isn't how it works; if the tool you use to do calculations is broken, you can't use that same tool to fix your broken calculations.