I switched from silk to Bottom Line and have been very happy with it's washability, performance, and ability to sink invisibly into the fabric. But you have specified you don't want poly, which is what Bottom Line is.
By the way - the reason I switched is because I could get one of those donuts full of pre-wound bobbins in a plethora of colors, instead of purchasing several different spools of silk. It was a much more economical option.