The really good, high poundage memory foam, at least 10 inches thick. Make sure you have a good, hard surface to put it on, like a hard box spring or plywood. Costco has a good one under the brand name of NovaForm. (I hope these links work, if not, just go to costco and search for NovaForm)
http://www.costco.com/Browse/Productgroup.aspx?Prodid=11168623&whse=BC&topnav=&browse=If you can't get the Memory foam mattress, but have access to a decent firm mattress to begin with, get the Costco NovaForm Choices mattress topper (very cheap). We got that one to put on our old Serta (still in decent condition) because we couldn't afford a new mattress to help my husband and his advancing arthritis and other pain issues that kept him from sleeping.
http://www.costco.com/Browse/Productgroup.aspx?Prodid=11134241&whse=BC&Ne=4000000&N=4001726&Mo=8&No=3&Nr=P_CatalogName:BC&cat=4257&Ns=P_Price|1||P_SignDesc1&Sp=C&topnav=
He's a big guy, too. He hasn't had painful sleep since. If you have pressure-point issues (hip pain, back pain, elbow pain) when sleeping, the memory foam mattresses displace your weight in such a manner that your weight does not focus on small surface areas in the mattress.
The only downside is that memoryfoam mattresses tend to hold in body heat. Good ones tend to have some sort of quilted topper to reflect body heat outwards.
Make sure you get the Costco version - not from BB&B, Sam's Club, or any other box store. Costco had NovaForm make a special run with higher poundage per inch on their mattresses, and a 2" foam pad from Costco displaces weight at the equivalent of a 4" foam pad at any of the other stores.
Haele