I'm getting into the DS again after realizing that this is a perfect little system to have on the crapper. If you're able to find a cheap system, most of the best games can be had under $10 and are easy as heck to store.
I can only give a Top 5 so far based on games I've played more than a few hours:
1) Contra 4
2) Super Princess Peach
3) Metal Slug 7 (good enough)
4) New Super Mario Bros.
5) Advance Wars: Days of Ruin (love the dark theme)
I found the Castlevanias and other lauded games like Mario Kart to be fairly meh the first time around and not really comparable to the mainstream console versions of the past. But I'll give a few of them a shot in the future since they're cheap enough and good to have on a road trip or at work while on a bathroom break.