There is no particular genre that I prefer but I do prefer cannon pairings. I will consider reading non-cannon pairings, but be warned that I will be much more critical of them.
As for the source of the story (movie, tv show, book, etc.), I have indicated the stories below that I know the most about. Someone who doesn't know the story very well is, most likely, not going to be as helpful as someone who does. For that reason, I have only indicated the stories that I know a lot about.
Supernatural is my number one story preference, since "obsessed" does not do justice to how much research I've done on the characters, cast, writers, legends, and even weapons and vehicles used.
Next comes Tithe, Hana Kimi, Escaflowne, Dark Angel, Harry Potter, and Firefly, closely followed by Everworld, 10th Kingdom, Stargate SG1, Howl's Moving Castle, Looking Glass Trilogy, and Doctor Who (from the 9th Doctor to now).
There are a few more stories listed below that I will beta for, but I am not as familiar with these as the ones mentioned above. I have read/watched these stories/shows, but there are some that I have not finished (Naruto, Digimon),
some I have not looked at in a long time (Princess Mononoke, Star Trek Voyager, Wishbone, Young Hercules, Sabrina the Teenage Witch, Titus),
and some that I have seen and thoroughly enjoyed, but have not had the opportunity to do any additional research on (Meru Puri, 10 Things I hate about you, Boy Meets World, Demon Diary, Fruits Basket, Gravitation)
Now that you know where my expertise lies for your particular story of choice, I'll let you decide whether you want me to beta your story. I will consider all stories sent to me, whether they are on this list or not. If I have several requests and limited time, however, I may turn down a story for something on this list.