For funsies, some stuff about the Buffyverse characters that I've learned from reading the books and comics (a.k.a. fandom things they like):
Willow is a fan of Roswell (the OG one, of course).
Cordelia likes Bewitched. Namely Endora.
Anya's a fan of The Jetsons.
Since in one of the tie-in books, Dawn makes a positive reference to Jessica Alba, I think this means that she's a Dark Angel fan (since when Buffy takes place, that's largely what Jessica Alba what have been known for, of course). We also know that Dawn is a fan of Pirates of the Caribbean from the comics. One of the books also let us know that she likes NSYNC. Edit: She's also a fan of Daredevil.
I'm pretty sure that Xander is probably a "Pokémon" fan, since in one of the "High School Years" comics, he goes into a comic shop where they also trade "Pokémon" cards there. Obviously, that's not the only thing going for it--as it's also a comic shop--but this just seems very Xander, doesn't it? And Pokémon was so big in the 90's, when Buffy largely takes place.
And in the Buffy seasons 8-12/Angel & Faith comics, a number of the cast were Game of Thrones fans... though I kind of forget which characters were (I think Harmony was one, but don't quote me on that). I should probably reread the comics someday to refresh my memory on this. And just to reread them, of course. LOL
Edit: And Willow and Tara both loved this show (that was made up for the books) with this live psychic, that Tara and Giles went to a live recording of (because something came up and Willow couldn't go, so Giles went with Tara instead). And we got this Tara and Giles bonding moment that was really cute. We should have had stuff like that in the actual show for dear Tara.
Edit 2: Both Buffy and Dawn are fans of The Princess Bride.
Edit 3: Buffy loves It's a Wonderful Life, and watching this film with her dad (a number of times) is one of the good memories she has with him.
Edit 4: Buffy may not like Twilight, because when Angel calls himself "Twilight" as an alias in season 8 (it's a long story), she's incredulous that this villain she's facing (she doesn't know its Angel yet) would call himself that (edit: I should probably specify that Angel is not calling himself "Twilight" because of the Twilight Saga. LOL. Like I said, it's a long story as to why he is calling himself that, exactly), and she says she did that whole thing (story) first and her vampire was cooler. It might not be so much that she dislikes it (though she very well could), but that she somewhat feels that Twilight ripped off her own story (LOL), and--like she said--that she thinks Angel is cooler than Edward.
On the subject of Twilight, in the "In Every Generation" novel--that admittedly isn't canon and in its own 'verse, like separate from the comics for instance--Frankie (Willow's daughter) talks about how her and her mom, and Buffy (and maybe some of the other Scoobies), actually enjoyed watching vampire things like Twilight and The Lost Boys--it's sort of a silly little hobby of theirs.
Edit 5: Angel possibly doesn't like the The Phantom of the Opera (musical), because this one guy in the tie-in books keeps asking him to perform in this "demons only" performance of it that he wants to put on, and Angel is annoyed by being asked to do this all the time. So it's either he doesn't like the musical or he's just tired by this line of questioning or a mix of both.
But Spike maybe likes Phantom, because he at least knows a little but about the novel, to be able to compare some tunnels that they go through in one of the tie-in books to it.
In one of the Boom comics (that is not canon), Buffy quotes "Mr. Brightside" by The Killers, so she's probably a fan of the song there, of course.
Edit: And in the comics, Xander makes a reference to the TV show "House," so I'm going to go out on a limb and say he's a fan of it:)
