I love a lot of them (including LotA like the OP), but my absolute favorite is Titan EX. It was difficult, but designed brilliantly to teach you the fight in manageable stages that built on one another, such that everything you learned in the previous stage helped you in the next, more complex one. The rewards for the fight were meaningful with both an additional set of end-game class accessories and weapons (through Primal Focus), which made learning and clearing the fight incredibly rewarding.

EX Primals these days just don't have the right sort of reward ratio, in addition to generally being a bit simpler, which makes them nowhere near as satisfying to me.