Hi jtstevanus,
The major brands such as Sealy/Stearns & Foster, Simmons, and Serta all tend to use lower quality and less durable materials in their mattresses than most of their smaller competitors that will tend to soften or break down prematurely relative to the price you pay which is why I would generally suggest avoiding all of them completely (along with the major retailers that focus on them as well) regardless of how they may feel in a showroom along with any mattress where you aren’t able to find out the type and quality/durability of the materials inside it (see the guidelines here along with post #3 here and post #12 here and post #404 here).
Tempurpedic mattresses use good quality materials but as you know they are in much higher (and mostly unjustifiable) budget ranges and for most people they certainly wouldn’t be in the best “value” range compared to many other smaller manufacturers that use similar (or in some cases better) quality materials that are in much lower budget ranges. You can read more about Tempurpedic in general in posts #1 and #2 here.
Don’t equate “best” with most expensive. What is “best” is not only based upon the quality of materials, but also the subjective part of your own personal value equation.
While the memory foam used by many manufacturers is proprietary, you certainly can opt to build your own DIY memory foam mattress. You can take a look at the different vendors in the component post here for foam sources if you desire such a project.
Memory foam and latex are very different materials, so I’m not sure what comparisons he’ll be able to make for you there.
Phoenix