Which Card is the best for you
If you want to play 3ds and ds games on your 3ds console, currently only gateway 3ds card can do it, you can check it here: Gateway 3ds, but gateway 3ds can only work on 3ds V4.1-V9.2, and does not support new 3ds currently, if you have updated your 3ds, you can not use gateway 3ds.
If you have already updated your system to the newest 9.5.0-22 J/E/U, go for the sky3ds card here. You will be able to play the newest 3ds games on your N3DS/3DS XL/New 3DS consoles.
If you only want to find a card to play ds games, do not want more advanced features, r4i gold 3ds card is always highly recommend. this card has very good game compatibility, you can use it to play nearly all the ds games, this card also updat from time to time to support the new games.
If you want to find a card to play gba games, currently only supercard dstwo card can do it, dstwo is the best and also the most expensive card, this card has some features other cards don't have, such as play gba games, slow motion, real time cheat, real time guide, real time save, if you need those features, buy dstwo card, if not, buy r4i gold 3ds.
If this is your first time to choose a card and i recommend you get r4i gold 3ds, it is a very user friendly card, you can set uo it in 2 mins when you receive it.