Your asking for opinions and ultimately you are going to have to figure it all out yourself based on what you want to do and what other commitments you may have.
If you need to go to Summer school, then you can rule out the advantage programs. If only certain classes are offered during certain semesters, then that would narrow it down too.
If you want to be around for the greatest number of special events that happen at WDW, Fall Advantage would be it. You may get the tail end of Flower and Garden at Epcot or Star Wars Weekends at MGM. You'll be there for the 4th of July when they have special fireworks. then in the Fall you have Food and Wine at Epcot, the Halloween parties at the MK, and then Christmas all over the place.
If you are wanting to be there when the weather isn't terribly hot and humid, then avoid advantage programs over the summer (it is almost always hot and humid, but June, July, August, are really bad).
Spring Advantage you would arrive when the parks are relatively slow and then get busier. Fall Advantage, you arrive when the parks are busy, then slow down, and get really busy again during the last weeks.
I won't say that any program is the best because what is "best" depends on an individual.
My advice is to look at everything as a whole and then move from there taking into account school, your family, and really what you want to get out of the experience.
