I thought HoD was a little meh, while Fusion just didn't do anything for me at all. I thought Zero was all kinds of better. I'm not a total nut for either series, but Castlevania seems to have made the GBA transition much more interestingly. I certainly won't debate that many feel otherwise.
I was disappointed with Fusion at first, but came to love it after my second try. The exploration part was done [relatively] poorly, but it was MADE for multiple run-throughs. Which is a bad thing, of course. Zero Mission gripped me immediately.