My friend (Not me, and I want to very clearly express this point
) had a classic DS with a flash cartridge. Shortly after getting it he let me borrow it. It had nearly every DS game in existence.
The ones I spent the most time playing were 358/2 days, Elite Beat Agents, and Crystal Chronicles. All absolutely superb games. I regularly see people bash the DS for a lack of good games, but, I really beg to differ.
Plus all the homebrew on it. Really an amazing and flexible device. I mention homebrew because there are quite a few really good homebrew games. You'll have to look those up. haha.