Okay, so before you all scream "Ultimecia" think about it for a sec. Edea inherited the power of an un-named sorceress at age 5, then the future Ultimecia's power a few years later. Both of these were later passed on to Rinoa when Edea relinquished her power. Rinoa also inherited Adel's power upon her death. Furthermore, there are hints that Rinoa's power will eventually be passed down several generations before finally reaching *gasp* a young Ultimecia.
Of course raw power isn't everything...Ultimecia does seem to be the most skilled in the usage of her sorceress power, whereas Edea is somewhat reluctant, and Rinoa...is just a mess. Though Adel's reputation seems to suggest SHE was the most rabid and demonstrative with her abilities...plus, look at her...she's 12 feet tall!
Another question: Why did Ultimecia feel the need to possess Adel, when Rinoa already possessed Adel's power (via an Ultimecia from a different time-stream?