I like to be able to permanantly learn my magic (which is the weak point of both the Materia and Junction systems), and out of all the FF games I've played, I favor the Esper system from FF6.

However, Kishi's explanation of Tactic's system has me thinking it'd probably get my vote if I had played more than a few hours of it. I really need to get my hands on another copy.

The Sphere Grid deserves mentioning. Spending hours trying to complete the grid is the only reason I played FFX so much.