I get what used to be known as an hotel quality innerspring. One that is flippable. Put on heavy duty box spring. And make sure the boxspring is supported fully with heavy wood supports across the area that your body rests. Doubles and queens can bow believe it or not.
Then purchase a latex foam non gel 3 inch mattress topper. Expensive.
Plan on flipping the mattress at least 2 x per year and the topper quarterly.
Replace topper about ever 3 to 5 years.
The hotel quality is the firmest possible.