To me, CotM embodies a good mix between the IGA era and the old-school-CV era. The difficulty is solid (even if the controls are not so much), the music quality is top notch (although most are remixes), and the DSS card system offers lots of flexibility. While the graphics aren't quite as polished as AoS', they're decidedly darker and moodier. People have made many comparisons with Super Castlevania 4, and CotM's soundtrack conisists of as much.
IMO, it's the best GBA experience.
While AoS was a cleaner, more balanced production, it also had some boring level design, or at least off-putting layouts. The music is much better than HoD, but most songs aren't as memorable as CotM's. And though the soul system was its crowning jewel, the act of trying to get them all would unbalance the game as you'd level too fast.
HoD was mostly a mess. The magic book system and dual-world was cool, but the level design made no sense (maybe on purpose), repeated too often, and the difficulty was low. One of the worse things about it is its presentation: the sprite design and animation sucks, even if there is some nice technical effects here and there in the backgrounds. The music is almost entirely forgettable. Despite that I give it credit for being unique and dark, it mostly grated on the ears and made levels feel too long and tedious. There's a couple of good tracks, but no standouts here.
CotM FTW.