You're pretty much going to get the same answers from everyone here.
The Supercard DSTwo, Acekard 2i, and Original R4DS are the best three flashcarts of their respective price ranges.
If you can afford the $35 DSTwo, it'll play anything you through at it flawlessly, it's that good. Not to mention the plethora of dedicated and unique functions it has.
Even if you only want to play DS games, DSTwo remains the best for its unbeatable compatibility.
The affordable $15 Acekard 2i is also great, whilst most games work, new releases like Pokémon Black/White might still need fixing through a patch or eventual firmware update.
It's a great solution if you just want DS games and are prepared to do a little work like rom-patching, or can afford to wait to play a new game.
Finally there's the cheap $6 R4DS, which has the custom Wood R4 firmware, regularly updated and very useful, but with the limitation of only 2gb max, and no DSi support.
Take your pick, depending on your needs these are ALL great.
