For me, it's a blood elf healadin that I created just after Burning Crusade. I've never quite gotten back to playing it after Cataclysm, but I think it will always be a favorite -- probably because I have great memories of playing with friends and generally having grand adventures on that character. New characters just don't have that history, and though I had other characters before and have had other characters since, I'll probably always be a paladin at heart.
What about you, dear readers? What are your favorite, most memorable characters, the ones you can't quite escape no matter how many new characters and classes you've tried?