Stop stop stop. The best on the market right now is, without a doubt, the Supercard DS Two. It comes with a built in CPU that emulates the way real retail cartridges work and it supports divx/xvid playback as well as SNES and GBA emulation. The team is also certain their card will work in a 3DS, enabling them to do at least something with it. It's around the same price as some other high-end cards (read: $30-35(I managed to snage one for 28€+shipping during a promotional at gamesyeah.com, but I'm sure there would be cheaper options since you live in america and don't need to worry about international shipping.)) but it's a very, very good investment because it'll bypass pretty much all piracy-checks there ever is and has been on the DS. Get one of those for maximum enjoyment.
http://gbatemp.net/t234082-the-supercard-dstwo-faq Very handy read if you're interested
I use an R4 DS, and that's been working pretty well for me, but I haven't really experimented with any other flashcards.
R4 is nice but out of date afaik and might have issues with the latest games.
With the release of R4 Wood it's actually one of the best flashcards out there right now because of constant updates to the unofficial firmware. It'd be fair to say it's about as good as the AKAIO firmware on the Acekard; the only real downside is that nowadays you'll only find clones in stores as the real R4 has been discontinued.
Anyway, you can get an Acekard for
$15 while the Supercard is around
$35, so it's really up to what you want from your card. If GBA emulation on the go is really important for you, go with the Supercard (it has limited compatibility for now, but most popular games are supported). If you want to play the latest games as soon as they come out it's also the best choice. If not, just go with the Acekard