Me and my father had this debate he is a bioshock fan and says it was game of the year 2007 while I say Mass Effect. Although I did played and beat both and they both recieved numerous game of the year awards.
I felt strange playing bioshock like I had to make myself beat it (still good though) while mass effect I enjoyed fully and was so upset that it was over that I had to play it 3 more times.
Although I do have to mention they both have incredible stories but mass effect's story is more appealing because you actually have conversations with people, but Bioshocks story is so damn ingenius and so intelligently written that I loved it as much.
I have to say Mass Effect wins in my opinion because it's more customizable, less linear, and feels generally more rewarding to beat and start over again with all armor weapons and skills.
Although at any rate play both it's only $40.
edit: thinking of Mass Effect vs. San Andreas I have to add that Mass Effect is sooo much better then San Andreas and that it is by far the weakest game of the series but GTA 4 would give Mass Effect a challenge because it's really fun single and multiplayer
