Cotton is the heaviest batting weight wise. Other things that will or can add to the weight is lots of piecing. The more seam allowances the heavier the quilt. so if you are looking for weight, choose a pattern with lots of pieces. Finally your backing can also add weight. Some flannels are pretty heavy.
You don't say how you are quilting it but if you are putting it on a frame and doing longarm you can add to the weight by making the quilt double sided and piecing the backing as well. But if your longarm balks at many layers of seam allowance you won't want to do this.
The heaviest quilt I have is batted with an old army blanket and I put a layer of poly on top of that to give definition to the quilting. Unfortunately the old scratchy army blanket is still scratchy even through the backing ( a quilting cotton ) so I only use it on the coldest of nights and when there is already layers of blankets and quilts between me and the scratchyness. I do like to feel that kind of weight on me when I sleep, especially on super cold winter nights.