If you want the best flashcart and don't care about the price: CycloDS Evolution
If you want the best flashcart but do care about the price: Acekard 2
If you want a flashcart with onboard NAND memory and if you plan on developing: Acekard R.P.G.
If you want a flashcart that also offers a Slot-2 (not for free though): EZFlash V & M3DS Real
If you want a Slot-2 expansion: EZFlash 3-in-1 (GBA, RAM, & Rumble)
If you want an alternative to the CycloDS Evolution but cheaper and a crappy GUI: SuperCard DS One
If you want a flashcart that could possible brick your Nintendo DS: R4 DS, R4 III & N5