symphonia still has the most spectacular boss encounters, the best music, the best art (characters and levels) and the most interesting castle layout -- including the inverted castle.
i don't get the love for aria, though. it was short, had a gimmicky castle layout, and a middling soundtrack effort. at best, it was short and focused, and the soul system was great for the ocd set, but i think i actually slightly preferred dawn. por spanks both of them soundly.