For me, this really is a difficult decision. Because I truthfully like the idea of both relationships.
Ciel and Lizzy would be absolutely adorable together, because I imagine how they would be if the anime were possibly set when the two were older. Ciel would be the series, mysterious and sexy type, a little like Sebastian but more empathetic and with the tendency to be cute at times. And I can see Lizzy risking herself for Ciel in any situation. HOW CUTE WOULD HE BE IF HE DID THE SAME FOR HER?!
Still, I am a sucker for Sebastian, Very much like, Grell, I practically worship the drop-dead sexy man. (Probably why I like Grell so much, we're very mcuh alike in that aspect and he's hilarious.) Ciel would totally be the unsuspecting uke type, and could you imagine just how double sexy Sebastian would be to whisper all too suggesting words to Ciel?! >///<
But, I may be being biast due to liking a little guy on guy action... redface
So, if we're talking a real relationship... I think Ciel and Lizzy would be better together to much of my dismay. Based on their personality, it would ruin Sebastian's character for me if he were to fall in love with Ciel. I enjoy having him being an all around demon with his respectful yet oh-so delicious sadistic and uncaring attitude. (Like how he had let Alois and Ciel fight without stepping in the stop it when he easily could have.)
So, Lizzy and Ciel get my vote.