If you like the feeling of sinking into your bed as opposed to resting on top of it, then a memory-foam mattress made entirely or mostly of polyurethane foam (aka polyfoam) may suit you best.
The Linenspa Memory Foam Mattress Topper is the best overall value if ... It's available in either a 2 or 3-inch profile – choose the taller one if you need more plushness.