I used an inner, 3 soaker layers of hemp french terry, and an outer. For inners I used organic cotton velour, organic cotton fleece, or a lighter weight organic cotton sherpa. For outers, I used the lighter weight organic cotton sherpa, or a heavy 19 oz sherpa.
I also made some contour shaped doublers out of scraps of the above fabrics. My fav combo is bottom layer of hemp french terry, two internal layers of sherpa, and a top layer of velour, although I may try micro fleece for the top layer of a night time doubler.
I would advise against putting three layers of hemp french terry as an internal soaker. This was fine in the winter when I made them up, but now that babe is here and it's a humid summer, I can't get them to dry. It is taking three cycles in the dryer! I could hang them on the line for days and they would never dry. Talk about a killer electric bill! I think my next batch will be pocket style.
I don't use flannel or interlock, as that just seems like more work to use all the layers they would need. And I haven't gotten around to trying BKT or terry toweling.
My favorite for any layer is the heavy 19 oz sherpa I got from a co-op. It is soft and thirsty and seems to dry faster because of it's loft. It is really pilly, but I don't care. It is thick, but my machine didn't have any problems sewing it, again, because it's not a really dense fabric.