Oldboy
On the face of it, this seems to be a typical Japanese kung-fu revenge flick - man is inexplicably locked away in a room by persons unknown for fifteen years, seeks retribution once released.
However, on the face of it, I'm an intelligent person - the truth can often be very different. This is, in fact, a Korean film (first one I've seen I think), and is highly recommended. I, it turns out, am a dumbarse.
The plot is pretty much as I outlined above, but with interesting twists here and there. This doesn't really sound that great, but this film stands out. To me, it seems that every part of the film has been created very deliberately.
I'm sorry I'm not writing this a bit closer to having watched it, or I'd go into more detail. Suffice to say, it's definitely worth a look, even if the plot or (broadly speaking) genre put you off.
Some parts are not for the squeamish, though.