Here's the real deal STK; clean of retarded RPG's and cheesy stylus stuff, I like games that make proper use of the dual screen, but unfortunatley not all games have it done properly, so if you like the creamof the crop then we're all set!
In no particular order
Contra 4 (Fuck yeah!)
Yoshy's Island DS (One of the first to use both screens nicely)
Mr Driller Drilling Spirits (Awsome title aswome game!)
Legend of Kage 2 (Super nice with cheesy dialogues here and there, but all ninja action the way it was meant to be; with no stylus!)
Ketsui Death Label (Here's a game that could have used the Dual screen)
Bubble Bobble Double Shot (Hell yeah! The intro alone pulls you in!)
Bomberman (the first one, you get to save the power ups and keep an inventory on the bottom screen for quick access, boss fights are dual screen!)
Littel Red Riding Hood Zombie BBQ (Wait 'till you play this, is goona be awsome!)
...and now my guilty pleasure (no stupid puns intended) Guilty Gear Dust Strikers: This game is amazing, but folks need to understand that is not quite like the other GG games, you need to learn the simple yet difficult mechanicsof the game to be profficient and have fun with it. There's one thing though, you need to up the difficulty 'cause otherwise is way to easy, I clean this game with over 5 characters no continues. I can only imagine 4 people playing this knowing what their doing and man it could be so much fun!
Think of this as a watered down GG Isuka, now if you didn't get or liked Isuka you still have a chance to like this as it works with levels on the dual screen rather than plane fights.
On the GBA if you want to play GBA games on it, you have the usual suspects, but I'll make a short super sweet list of the best of the bestest!!!!
Ninja- Five 0 (Greates game in a handheld)
Gradius Galaxies/Generations (Hell fucking yeah!)
Aladdin (Al kinds of amazing like it's SNES counterpart)
R-Type III (Man this is it right here!)
Super Street Fighter II Turbo Revival (Actually is pretty damn amazing for this game to fit so well in the GBA, man is like the SNES SSF2, but better! Way better!, the buttons take a bit to get used to, but once you're there you'll be fine)
PS: I didn't mentione SNK games 'cause the 2 KOF's for GBA are ok, but no cigar, the GGX for GBA is just not as fun as the WS ones and Metal Slug 7 is cool, but I haven't played enough to warrant a recommendation. I also didn;t include any Metroids or Castlevanias or the susal suspects 'cause I'm trying to keep it clean here, mostly aracdey stuff which to me is the way all handheld games should be.
I don't think any SNK games in handheld are ever gonna top NGPC, there's just no way in hell that can happen. oh! The SVC card game in DS is pretty cool, but is RPG'ish so I didn't think you'd be interested.