Harry Potter- because I love Harry Potter
Star Wars- just look at my name and avatar
Heroes-I love Heroes, and I love Twilight, need I say more
(Yes, pretty much my reason for all of my crossings are because I love the series, but why else would I cross two fandoms?)
Also, I've just started reading this series, Maximum Ride by James Patterson, and it's really good. I just finished the third book, and I think it would be a very cool cross with Twilight.
Oh! This isn't a series, but I want a Dr. Horrible's Sing-a-Long Blog crossover with Twilight! If someone knows of one, please point me in its direction, and if there isn't one, what are you waiting for? Go and write one!