Hi KeepCooler,
Memory foam mattresses as a category will tend to sleep warmer than other types of mattresses and I don’t know of any specific or legitimate testing that has been done between different memory foam mattresses that has measured the actual sleeping temperature for a wide range of people in any scientific way so I don’t think that anybody can tell you which memory foam mattress sleeps the coolest.
There are also many factors that can affect the sleeping temperature of a memory foam mattress (or any mattress) besides the memory foam itself and there are also differences between different memory foams and mattress designs that can make a difference so for most people if a mattress sleeps “cool enough” or as long as they are in a comfortable temperature range for them that’s all that is really important to them.
Post #2 here has more details about all the different factors that can affect sleeping temperature and post #6 here has more about the differences between different types of memory foam and memory foam mattresses so you have some reference points to look for that can help you compare different memory foam mattresses in terms of their “sleeping temperature potential” in the absence of any specific information that can tell you exactly how warm you will sleep on any specific mattress.
Phoenix