Hmm, old topic, but ah well. You probably have purchased all three, but for the help of others, i'll give my 2 cents. I have all three, and lean more on Mass Effect and Bioshock than Oblivion. Many people probably will disagree with me, but, Oblivion and Fallout are the same game to me. The only difference is the years and the weapons. Personally, i enjoyed Fallout more than Oblivion (one of the few times i enjoyed shooting more than slashing). As for Mass Effect, i'm currently playing through it, but i like just about everything. My one qualm so far is when you go to Noveria, you have to fight an Adept (basically wizard) and a swarm of her followers, and there are long dialogues you have to go through to get to the fighting parts, and i often died right after the conversation, so, that got annoying. Otherwise, i really liked every part of Mass Effect. Bioshock, to me, is one of my all time favorite games. Its very dark, at certain parts its frightening to play at night, and a wonderful scheme

Very bloody, as well

Story isn't exactly the longest, but it kinda throws you for loops and makes the plot come on as a little longer every time you achieve something

Personally, my vote is Bioshock (note: it's a Sci-Fi FPS, so that might be a turn off)