If you c an get ahold of Aria of Sorrow on the GBA (or just emulate it) do that, then play Dawn of Sorrow on the DS (Aria and Sorrow follow the same set of characters). As others have mentioned, Portrait of Ruin on the DS is also a great Castlevania.
Others have mentioned Symphony of the Night (SotN) the only trouble with it is it is a rare game these days, and being the first Metroidvania, is waaaaay of the easy side. If you happen to be one the poor suckers like me who bought a PSP, you can get an upgraded version of SotN on the Castlevania Rondo of Blood remake for the PSP. SotN, as Rondo's spiritual sequel, is ported on there with a few upgraded features.
If you want to start at the classic roots of Castlevania, Dracula's Curse on the NES is considered among the best of the classic but **** is it *hard*!
3DS FC: 5413-0163-7657 Name: Randy Please tell me if you add me so I know to add you!