The two best types of topper pads for beds are those made of memory foam or those made of latex foam. Both have their own advantages and disadvantages. Which one you buy depends on what you can find and what your real needs are.
One important disadvantage of the memory foam topper is that it is temperature sensitive. It becomes softer when warm, and becomes firmer when cool. Although this feature allows it to mold itself to your body, giving unsurpassed comfort (compared to other materials), it also means that a memory foam bed topper that is just nice in summer may be too firm in winter, and vice versa. This may not be a problem if your bedroom is always the same temperature when you sleep. If not, you may want to think about buying a latex topper for your bed instead.
One problem with the latex mattress topper is that there are different types of latex foam. To be more accurate, there are different ways to make the latex foam, and different production processes produce foam of different quality. The best are supposedly made using the Talalay process. If it is your intention to buy a latex foam topper, you may want to make sure the label says it is made using the Talalay process.
If you have hyper-sensitive lungs, you may want to give both the memory foam topper pad and the latex topper pad a miss. Both of them are known to emit noxious chemicals, albeit in very small amounts that will not affect most people. In the case of the memory foam pad, these fumes are classified as Volatile Organic Chemicals (VOC), part of which comes from the production process, and part of which comes from the very slow but continuous and natural deterioration of the memory foam itself. In the case of the latex mattress pad, the chemical is ammonia, left over from its production. Some high-end foam toppers claim to remove these chemicals through a special washing process. Whether or not these claims are true, whether or not these procedures actually do any good, is something that is not clear.
You may be better off with a mattress topper made of all-organic materials. A topper pad made of organic linen and filled with organic cotton may not be as good as a memory foam topper, but if you make it yourself or have it custom-made by a tailor, you know that everything is natural and does not contain potentially harmful chemicals. You may be tempted to buy an “organic” bed topper that is mass-produced. However, do be aware that many regions have laws making it compulsory to treat all bedding products with flame retardant chemicals. If you have a problem with these chemicals, having your bedding hand-made may be your only choice.