All three are excellent options, but like Bam said Mass Effect 2 is more of a hybrid RPG/Shooter. Bioshock 2 is an excellent game, but feels like more of the same if you’ve played BioShock 1. Metro 2033 is a game I have some issues with but still enjoyed. It’s brutally difficult for one, and brutally inefficient as well.
The difficulty curve is massive, pitting you against creatures in the dark for one moment, then fully armed neo-nazis the next. As for being inefficient, it’s a system hog. STALKER, Clear Sky, and Call of Pripyat all suffer from similar issues to Metro since they all use the same tech. They run choppy and sluggish on all but the most powerful machines, especially if you want to use DX11.
I’d say go for Mass Effect 2, it will entertain you for a very long time, provides a lot of re-playability (which BS 2 and Metro 2033 do not) and is just plain fun.