Many enjoyable films posted already. No one has mentioned 'Primer' (2004/Shane Carruth). Respectable as hard sci-fi, and inspiring as an independent project (Carruth did almost everything himself.) I suggest watching before you read anything about it.
For non sci-fi, 'Old boy' (2003/Chan-wook Park) was memorable - fun concept, well acted and a good ending too.
For non sci-fi, 'Old boy' (2003/Chan-wook Park) was memorable - fun concept, well acted and a good ending too.