I have to say that Bioshock is almost definitely the best singleplayer game i have ever played. The game just continues to amaze me and i keep finding things that i have never found before and doing things that couldn't even imagine even though i have almost completed it twice.
The graphics are stunning, and the fact that the game is set in 1960 means that there is plenty of colours and great and inventive designs and so you never get bored of the amazing scenery. And i can't mention graphics without saying how goddamn outstanding the water effects are.
The storyline (don't worry there'll be no spoilers here) is so good with twists and new plots everywhere that keeps you guessing and makes you want to play it even more to find out what happens; you can really tell how much effort they have put into developing the storyline and adapting it to the other creative aspects of the game.
I don't think there'll ever be another game that is quite as original as Bioshock for a very, very, very; long time.