Bumgenius, or any other with microfiber. It's just too much of a headache to wash them.
Actually I probably shouldn't even answer here, because I've become such a flats snob. I got some free Babykicks Hemp one-size diapers that are pretty amazing, but never use them because they're a pain compared to the flats.
We started with 3 dozen flats from Green Mountain Diapers (they now have organic). We snappied them under a Thirsties as a newborn, or a Imse Vimse wool cover.
Now we just pad fold the flats and put them in a Flip, Imse Vimse Wool, or G Diaper cover.
Everything else I have tried collects stink, or needs complicated washing routines. I've given away a few hundred in fancy diapers because I so love my flats.