Original R4DS, Acekard 2i and Supercard DStwo are all good, but have different things to offer.
They all have great rom compatibility, can load Moonshell, and support the EZ flash 3in1.
Anyway, if you're looking to buy a new flashcart here's some advice.
The
R4DS is super cheap, it's only $6 and with a 2gb microSD, you can fit in some 30+ DS games. When you install the custom Wood R4 firmware, it'll play games amazingly well. The only downside is that you're limited to 2gb, and it has no DSi support.
(Also, only the ORIGINAL R4DS supports Wood, none of that SDHC/R4i/Gold crap)
The
Acekard 2i is also very affordable at just $15, with AKAIO firmware it has great compatibility and the added benefits of DSi support and SDHC support (so more than 2gb memory).
It'll work on DSi 1.4 but you'll need to flash it for it to work.
The only issue really is the contacts issue, which is easily resolvable.
Finally, there's the
Supercard DStwo, it's arguably the best flashcart, at a price of $35.
The DStwo has an onboard processor that is capable of GBA emulation, SNES emulation, video playback, and more, and there's an open SDK that will allow developers to make exclusive enhanced homebrew for the DStwo. In other words, DStwo has the best emulation potential of the entire DS homebrew scene.
The DStwo has an advanced real-time menu that you can access on DS games, where you have real-time saves, toggle cheat codes, enable slow motion, and check game guides in .txt or .bmp format.
So, those would be the best flashcarts in my opinion. To sum it up:
R4DS - cheap and effective, but limited
Acekard 2i - Affordable and works very well, a standard flashcart.
Supercard DStwo - A premium and pricey flashcart that performs very well.
Remember to avoid fake R4s like R4 SDHC, R4i or R4 Gold, they won't be anywhere near as good as the cards I mentioned here.