I always thought that the "Metroidvania" term referred specifically to the Metroid-like Castlevania titles. I thought it was supposed to be a clever and convenient way to distinguish the post-Symphony of the Night releases from the classical, more linear style of Castlevania. Now I'm hearing that any game of that Metroid-style action-adventure formula may be defined as a Metroidvania. Thus has the press dubbed Shadow Complex a Metroidvania. Moreover, apparently the Metroid games themselves are Metroidvanias. Doesn't make a lot of sense to me, but so be it.
In any case, the one "Castleroid" that I might consider superior to Symphony of the Night is Castlevania: Aria of Sorrow for the Game Boy Advance.
The third GBA Castlevania, Aria of Sorrow reunited the Symphony of the Night team of series producer Koji Igarashi, artist Ayami Kojima, and composer Michiru Yamane. In other respects as well, this was the true successor to Symphony of the Night.
The protagonist, Soma Cruz, was not a whip-wielding Belmont but, rather more like Alucard, an adaptable character capable of wielding whatever manner of weapon he encountered at random in Dracula's castle. In fact, Soma's arsenal offered even greater variety than Alucard's.
Excepting novelties and special weapons such as the Crissaegrim and Estoc, Alucard swung basically all of his swords with a quick horizontal slash. He would not alter his one-handed swing even for swords explicitly defined by the game as two-handed, such as katana or the massive Zweihander. Other weapon classes, including knuckles and flails, offered only slight practical variety while seeing less use overall.
Soma, on the other hand, favored vertically swinging broadswords, but he could also wield shortswords, spears, axes, hammers, unique weapons like the whip sword, and even firearms. And the type of weapon equipped had a real effect on how Soma played; although he swung the axes in a similar overhead manner as with swords, a heavy axe was much slower than a sword and had a more particular attack range, as only the blade was actually capable of hitting.
Some of the cooler weapons could only be won through the unlocked "Boss Rush" mode, but the main adventure still treated the player to a steady mix and flow of items to toy around with. For comparison, I spent about three-quarters of the sequel, Dawn of Sorrow, wielding just the Claymore while feeling frustrated and impotent.
Perhaps Aria of Sorrow featured fewer obscure items to hunt down than Symphony of the Night, but it instead offered you the souls of your defeated enemies as dropped loot. Consolidating Alucard's messy mix of heart-powered traditional sub-weapons and MP-powered spells and abilities into a single unified system, soul-absorbing allowed Soma to equip the powers of his foes, including such classics as the standard skeleton's bone toss. Because almost every enemy had a soul to collect, Soma's list of abilities could grow quite massive, and in Pokemon-like fashion, the drive to catch them all could keep players going long after the final boss was beaten.
Even Aria of Sorrow's story was the most ambitious since Symphony of the Night, though perhaps not ambitious enough. Set in the near future, Aria of Sorrow took place after Dracula had supposedly finally been slain once and for all--a moment not yet depicted in any game--though trouble was clearly afoot as Dracula's castle rose without its master.
Getting rid of Dracula as a character was a gutsy move, but some Castlevania traditionalists were less than enamored of his successor. At the same time, the game completely squandered the future setting, as Dracula's castle and everything in it remained distinctly medieval. Nevertheless, the twist-filled story was once again a compelling part of the adventure.
Where Aria of Sorrow most improved upon Symphony of the Night was in the level of difficulty. Neither as pitifully submissive as Symphony of the Night or Harmony of Dissonance, nor as cruelly punishing as Circle of the Moon or Order of Ecclesia, Aria of Sorrow achieved nearly the perfect balance. Soma was not nearly as godlike as Alucard, so bosses could pose real threats. It was quite possible that you would encounter the Game Over screen a few times, but then you could usually come back and persevere with a better strategy or just by being more careful.
There were still stats and level-ups, but progress felt more subtle. Unless you were going out of your way to hunt for souls, you would usually feel at just the right level for where you were in the game, so that victory was more often a matter of player skill than character stats.
Dawn of Sorrow included the best "bonus character" mode by far, while Portrait of Ruin featured a cool two-character system and a hero who could wield whips as well as swords. Both are fine representatives of the Symphony of the Night platform-adventure-RPG paradigm, and even Circle of the Moon and Harmony of Dissonance were excellent games in their own right, but in my opinion Aria of Sorrow remains, from a gameplay perspective, the definitive Metroidvania Castlevania experience. Even so, I can't say it was as significant as Symphony of the Night, either historically or personally. Three iterations after Symphony of the Night, the formula was still addictive, but it was no longer groundbreaking.
Perhaps equally important, even disregarding historical context, Aria of Sorrow, due to reduced budget and/or ambition, just does not achieve the same impact as Symphony of the Night, which features fully voice-acted dialogue, pre-rendered cinematics, and one of the most gripping beginnings ever to make it feel like a major affair. Aria of Sorrow boasts excellent production for a GBA game, and it still has Kojima and Yamane, but the handheld experience is just not the same. The gameplay is more refined and as fun as ever, but it does not carry the same weight, and thus it feels more like a diversion to pass the time than an event to set aside time for. Nevertheless, if you have room for only two Castlevania titles, Aria of Sorrow should be one of them.