I've mainly used WSP and Brambleberry so far, but I'm hoping to put together a Nurture order when I whittle away my current collection of FOs. I've heard very good things about Nurture's FOs.
I like that WSP does free shipping over $40 and they have a huge, easily sortable, selection. A lot of it is hit or miss, but if you only choose FOs with lots of shining reviews from CP/HP soapmakers, they're a pretty safe bet.