Aria of Sorrow all the way for me, Pfil!
I replay it often (mostly during holiday flights!) and I just adore it's mystical-like atmosphere: the washed out colours and palette, it's mysterious storyline and characters, areas with fewer enemies contributing a sense of desolation, the at times eerie and disjointed music that enhances the mood and level of immersion. This is not the grandeur or richness of Symphony but my goodness it has a unique atmosphere, and one that deeply appeals to me.
I could gush over the Tactical Soul system. It's ingenious from every angle: the way it was worked into the story with the castle monsters and their progenitor reborn; the brilliant, uncomplicated control and selection mechanisms in the game menu; the sheer number and scope of available souls--creating not only a novel means of advancement through the castle, brilliantly tied into events and Soma's progress, but offering weapon and status enhancements too; random drops set with careful deliberation to encourage maximum replayability.
It rarely frustrates, it oozes class, and breeds reward and satisfaction.
There is filler of course, but IGA and his team created something magical. What helped it considerably I feel, and where I think it's successor failed, was the aforementioned atmosphere and the game's outstanding castle design. I find it flows with brilliance. Not merely the simple 'stacked'/copy-pasted rooms of Dawn or Portait, as level-design began to stagnate and fall away. Ecclesia's final castle does Castlevania level design at its absolute worst, years after this gem.
If I were to be critical of Aria though, perhaps it feels on-rails at times.