Which rabbi do the majority of sefardim follow? Or, do a majority of them not follow a single rabbi?
As of May 3, 2010
While not all Sefardim follow him all the time, his word is definitely given a great deal of weight. Is that who you had in mind?
The Shulchan Aruch (written about 450 years ago) is usually used as a framework for Halacha today. Within it, Ashkenazim usually follow the notes of R' Moshe Issreles, but Sefardim follow its primary author, R' Yosef Karo.
As mentioned by Shalom, Rav Ovadia would probably be called the "Sefardi rav." Other Sefardi rabbis that people also follow such as: