Hi E.J.M.,
As LJGMDAD mentioned “it depends” is the only meaningful answer to your question. What it depends on would include many variables such as your body type, your sleeping positions, your preferences, your sensitivities, and others. In addition to this for most people a mattress that works best would usually have multiple layers that are different ILD’s not just a single layer.
I would always keep in mind that you are the only one that can feel what you feel on a mattress and there are too many unknowns, variables, and personal preferences involved for anyone to be able to predict or make a specific suggestion about which mattress would be the best “match” for you in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences) based on specs (either yours or a mattress) or “theory at a distance” that can possibly be more accurate than your own personal testing or sleeping experience (see mattress firmness/comfort levels in post #2 here). In other words a mattress that is “perfect” for one person may be completely unsuitable for someone else to sleep on so there is no such thing as “ideal” in general terms … there is only “ideal for you”.
In your other topic you mentioned that you wanted to build your own bed and if you are attracted to the idea of designing and building your own mattress out of separate components and a separate cover then the first place I would start is by reading option 3 in post #15 here and the posts it links to (and option #1 and #2 as well) so that you have more realistic expectations and are comfortable with the learning curve, uncertainty, trial and error, or in some cases the higher costs that may be involved in the DIY process. While it can certainly be a rewarding project … the best approach to a DIY mattress is a “spirit of adventure” where what you learn and the satisfaction that comes from the process itself is more important than any cost savings you may realize (which may or may not happen).
If you decide to take on the challenge then I would either use the specs (if they are available) of a mattress that you have tested and confirmed is a good match for you in terms of PPP as a reference point (the same type and density of foam in the same thickness and firmness levels and a very similar cover which can also make a significant difference to the feel and performance of a mattress) or use a “bottom up” approach (see post #2 here).
Phoenix