I'm currently playing Mass Effect 2 and so far really like the story. One of the aspects of games like this is the relationships your characters develop with some of the game characters. They always remind me of Mad Libs books that you can use over and over again and get different results. Unfortunately, I don't have hours and hours to play over and over again to find out about romances between various characters.
So with that being said, here are the spoilers with a big help from Games Radar, who have already put out some excellent walkthroughs.
The list starts off with male Shepard, as he's a manwhore and everyone knows it. Followed by female Shepard who is a downright freak and that's why she is beloved. There are two other options but are too much of a spoiler.
All videos pretty much cut right to the chase, but a link has been provided for the guide.