Definitely Dracula. He is a classic vampire figure and can be used in all kinds of vampire transformation scenarios. He could be the one doing the transforming or be the reason someone else transforms. Also, Stefan Salvatore from 'The Vampire Diaries' is popular. His struggle between his good and bad sides can be explored in the context of a transformation story. And let's not forget about Carmilla, she has her own unique charm and backstory that can be incorporated into fanfiction about vampire transformation.
Edward Cullen from 'Twilight' is often a popular choice to be involved in vampire transformation fanfiction. His brooding nature and complex relationship with Bella make for great story material. Another one could be Lestat from 'The Vampire Chronicles'. His flamboyant and often cruel personality can add a lot of spice to transformation stories. And then there's Spike from 'Buffy the Vampire Slayer', his bad - boy image and his own transformation story in the show can inspire many fanfiction ideas.
One popular character is Alucard from 'Hellsing'. His powerful and mysterious nature makes him a great addition to vampire transformation fanfiction. Writers can play with the idea of him either causing or preventing a transformation. Another is Selene from 'Underworld'. Her story of being a vampire and her battles can be intertwined with the concept of transformation. Additionally, there's Angel from 'Buffy the Vampire Slayer' and 'Angel'. His redemption arc and his vampire nature can be the basis for some really interesting transformation fanfictions.