Not sure about the best, but the most powerful movie I've seen lately is a French film called "Irreversible". If you want a *POWERFUL* film, which will run you through several different emotions yet will leave you feeling happy, you can't go past it (it IS a pretty horrific/disturbing film though).
(Stolen from: [www.imdb.com
If movies are suppose to effect the viewer then this movie is king of the hill. From the dis-orienting camera moves that open the film through the violence in the gay bar, the seemingly never ending rape and the films unraveling of the days events in a backwards march I have yet to meet anyone who has not been deeply effected by this movie - if they were brave enough or stupid enough to watch it. (I'm still trying to figure out a local Best Buy displaying a large number of DVD's as if it was the latest Adam Sandler movie)
It is not an easy movie to watch but as an examination of what people are capable and how violence can come from nowhere and from the unexpected its a masterpiece. The backward structure takes what is essentially a dull revenge story and turns it on its ear as we are forced to really examine how things get out of hand very quickly and what people will do when pushed to the limit.
If you want a challenge to your psyche and can brave simply some of the most vicious and nasty screen violence (yes its graphic but much is also implied) then see this movie - preferably with out distraction. But be warned, even if you think you know what this movie is about and what your limits in screen violence is, you still won't be prepared for what a mental hot foot this film is.