I personally recommend getting a DSONE, although they seem to have a high DOA failure rate. But imo it's the best cart available right now since it has a decent kernel and exclusive features.
Second choice would either one of the Ace3DS carts (Plus or X - they're both pretty much the same minus Ntrboot afaik). This uses an outdated kernel (certain games with AP will have to be pre-patched), but it's a widely known brand and on the market right now.
Third choice would be the r4isdhc.co.hk and r4isdhc.co.cn carts (they're both the same). Also uses an outdated kernel like the Ace3DS. Not much difference functionality-wise from it, either.
Last choice is the r4isdhc.com, solely because it has a time bomb in the kernel (can't be used beyond a certain date), although this can be bypassed by using YSMenu instead.
Honorable mentions:
Acekard 2 - Great kernel, but no longer being sold; even clones are gone (although there is a listing we think may be an Acekard, but not exactly sure).
DSTT - Okay cart, but not really worth buying over an DSONE, Ace3DS or even a r4isdhc.co.hk cart - No longer being sold (although it is currently for sale in limited quantities).