I love Green Mountain prefolds. No folding down! Woo!
My real preference is something other than traditional prefolds though. I'm making bamboo velour prefolds, but I also like the idea of other bamboo fabrics, cotton velour (though I heard this gets stiff with washing?), organic cotton (the flip OC inserts look awesome! I think I'll try my hand at making some of those type too. I think its a heavy cotton jersey?), etc.
There's a huge variety out there!