Castlevania: Portrait of Ruin
The action doesn't ramp up much - which isn't to say that PoR ends up disappointing; it just starts out stronger than most games are willing to. The dual-character system is brilliant, and the spells and techniques are a ton of fun, too. Weapon customization isn't as big a factor as in Dawn of Sorrow, but is still around. And, while not of the tremendous length I was hoping for, Portrait of Ruin is still probably the longest Castlevania I've played.
Of course one of the coolest parts of any Castlevania nowadays is the extras you unlock after beating it, and PoR has a ton to offer there, too. In the game you fight two vampire sisters, and an extra mode allows you to play as them: this mode isn't heavy on customization, but is played exclusively with the directional pad and stylus/touchscreen. There's also an extra mode for playing as Richter Belmont and the young girl Maria, which is played just as a classic Castlevania game (like Julius Mode from DoS). There's even a secret unlockable Old Axe Armor mode, which allows you to play as one of the game's enemies, Old Axe Armor.
All told, I will say that Charlotte's spell-switching system felt cumbersome at times, and I was a little let-down when the fifth through eighth portrait worlds turned out to be very similar to the first-fourth ones. Nonetheless, the game is fantastically fun and pleasing, and having played all the GBA and DS Castlevanias, I'm comfortable in saying that this is the best one yet.
[edit] Oh, and I forgot to mention the WiFi modes; there are two, a co-op Boss Rush run (one player as each partner), and a Shop Mode (which doesn't work exactly as you might imagine, but is still ingenius). They work great and are welcome additions to the game.
Progress: 1000%, finished most extra modes