I don't really ship anyone with Ciel because in my mind Kuro ends with him dead. There's just no other way.
However I do think Sullivan is a better partner for him than Lizzie on the grounds that:
1. She's of a better social standing now that the Queen supports her
2. She's actually a very smart scientist, so she can hold her own against him. They could make a good pair based on intellectual compatibility itself.
3. She knows he's done and dealt with shady things, that he deals with death all the time. She can stand by him instead of blaming him
4. Even if they don't fall in love, Kuroshitsuji is all about how Ciel uses everyone as a chess piece to get his ultimate goal of revenge. She can be his queen, his Trump card that he can use in the society to better back him up as the rightful heir of the Phantomhive AND use her against the Queen herself if the time ever arises.