You know, even bad Castlevania games have their moments, but after much consideration..."Curse of Darkness."
In the past I've called it the "Curse of Mediocrity." On the plus side, it had a few intriguing characters (who didn't quite pan out), it tried to create an interconnected world structure in 3D, it had some great tunes, and added a functional player-controlled 3D camera. HOWEVER, the game itself is one I have tried and cannot play through a second time, even with Trevor mode. It is so huge, empty, and devoid of platforming. LoI had better level design, and it had weak level design. In fact, CoD is almost a step back from LoI in every way: Unneeded, Chaos Legion/Pokemon elements were added as the main hook to cover for the deletion of 99.9% of platforming; the whip was nowhere in sight; and all the story build-up of Leon from LoI was dropped in favor of an inconsequential side-story that made Trevor look like a fool.
It was just more evidence of all the many things CV64/LoD got RIGHT. There's no way CV64 is at the bottom of the pack; at the very least, it's in the middle. I think it gets such a bad rap because of its timing with SotN and people expecting that style to be the main style. That aforementioned PS1 game, despite its quality, has gotten a little overblown over the years IMO. But I digress...
A few more comments: CotM had a classic CV feel and challenge to it even as it took the Castleroid route and was the first experimental outing on GBA. CotM just might be the best GBA or DS Castlevania overall. PoR was repetitive and goofy at times, but I found it a fresher experience than DoS, which was like an expansion pack of AoS. The original GB Adventure has tough controls, but it has a lot of unique charm and fun to it that I feel it gets more static than it deserves. Don't get me started on Dracula XX/Vampire's Kiss, because I love that game. Better than Rondo, I think, as it's more focused, more challenging, and has a better visual & musical atmosphere. I cannot speak for the pre-NES Castlevania games, as I have never played them. Also, OoE I skipped, as I had enough of the meandering "Castleroid" with no death pits style of games. I'll come back to it eventually, like I did for DoS and PoR. Oh, and didn't bother with Order of Shadows, because it was a cellphone game and looked boring (no offense).