I'd have to go with Lament of Innocence. I remember playing Devil May Cry back in 2001 and thinking "This is what they should do with a 3d Castlevania!", then getting Lament of Innocence and realizing that essentially that's what Konami did, and the result was incredibly disappointing. Lament for me forsook almost everything I liked about Castlevania. There was little detail, the enemies were repetitive and lamely designed, the gameplay was monotonous, the story uninteresting; it was a huge letdown. It featured neither the great gameplay of the classic, pre-SoTN 'vanias or the RPG-esque collecting/forging/magic/souls features of the GBA and DS varieties. The music was good, and the atmosphere is right, but everything else left me cold.
Curse of Darkness suffers from many of the same issues, but it was an improvement from my perspective as the weapon/armor forging, stealing, Innocent Devil familiars, and even the silly chair collecting made the bland combat more forgiveable. The enemies in CoD are still not up to series muster, but there was a greater variety to them, and the backgrounds and level design, though still wanting, were also improved. Storywise, however, CoD is just plain silly. St. Germaine is easily one of the most pointless and unnecessary characters in all of Castlevaniadom.