The R4i, the Acekard2i, and theres a 3rd but I don't recall the name.
You want the Acekard2i, it gets the most support by far. It seems to work the best with newer games to. The R4 may be tempting because you know the brand name but its not made by the same company as the previous R4. They just used the name to make people think that. The acekard 2i has 2 different firmwares offered. The official one, and then aikko the unofficial. You will want aikko but you will have to update it with the official first. And to update the first time you need a ds light or ds fat. You will also have to google the "Danny Phantom Update" It is what makes it work with the xl. It makes it think the flash cart is the real danny phantom game and thats how it loads it.