I haven’t played most Castlevania games myself, I mainly know the DS games, and played two of them like 10 years ago, Portrait of Ruin and Dawn of Sorrow. I remember them being pretty good. The third DS game, Order of Ecclesia didn’t work for me back then, because of anti-piracy stuff. Any of those three games should be fine on the 3DS (Dawn of Sorrow is a sequel to the GBA game Aria of Sorrow, but I don’t think it really matters plot wise)

This is actually why I got the Advance Collection and the more recent Dominus Collection, because I wanted to go back and check out a few of the games I missed and re-play the DS games, to see how well they held up.

If you hacked your 3DS, you can of course also try games for other systems, like the GBA games (mostly for the aforementioned Aria of Sorrow) or maybe even Symphony of the Night, which supposedly runs fine with some tinkering.

If you’re not into the whole Metroidvania stuff and want more classic, linear side scrollers, then the old NES/SNES games are also available somehow (but maybe not anymore, unless you’re doing homebrew stuff). The standout here is probably Super Castlevania IV, but tbh I never really played these myself.