depends on the quilt...i love wool batts and use them in the 'big- very special quilts..they are expensive. I use cotton alot...just like to work with it too...and i use poly usually for the "utility" quilts...kids quilts ; quilts that will be used and abused and washed/dried alot....the cotton and wool batts wash up wonderfully too but i like the poly for kids quilts it seems to hold up very well and takes a beating.
and for the really really special quilts the silk blend is OMG wonderful!