Im gonna say the bioshock series, incredibly atmospheric, excellent game. Bioshock 2 might be looked upon as an unnecessary sequel to a complete game, but stand alone it's still quite good. Also Bioshock 2 gave us the DLC story Minerva's Den, a 3-4 hour romp, with an incredible ending, an ending which i think is greater than the 'reveal' in bioshock 1.
Also, another excellent sci-fi series is dead space 1 and 2, very scary and very good, the atmosphere taking elements of every sci fi horror movie from modern times, particularly the alien franchise. Again, there is some story DLC for it, and again it's quite good.
So, yes, i completely recommend those two franchises.
*wish we saw story DLC for the halo games*