There is no absolute best flashcart: everyone has their own needs.
We recommend you do a little research, read reviews...
Currently three cards seem to stand out though.
- Supercard DSTWO: $35, best compatibility, GBA emulator, SNES emulator, DivX player...
- Acekard 2i: $15, excellent compatibility, AKAIO
- R4: $6, very cheap but awesome thanks to Wood R4