If you are even mildly interested in castlevania, you must own Castlevania Dawn of Sorrow. Its the greatest castlevania game ever to be released in America. (can't comment on Dracula X cant get a Turbografx emulator to work, and I don't feel like payings millions for the game on ebay ) That's probably my favorite game on the DS. Portrait of ruin is pretty good also, but it has some boring levels.
If you like Mario 64- you need to get its remake for the DS as they added a ton of new content to it - you can use the stylus to control mario and it works pretty good.
Metroid Prime Pinball is my favorite pinball game, ever. it uses the stylus to tilt the machine, and that works pretty well. It has awesome levels, and comes with a rumble pack. Its also pretty cheap.
Kirby's Canvas Curse - This is a platformer that uses nothing but the stylus. It has awesome levels, and is the best kirby game ever. You must pick it up.
Mario Kart DS - Doesn't use stylus, but it is a great Mario Kart game with tons of great levels and online play!!!