Structural Support (The Mattress)
There is no universally "best" mattress. There is only the correct mattress for your specific mass and primary sleeping position.
A mattress must accomplish two contradictory tasks simultaneously: it must yield to protruding body parts (shoulders/hips) to relieve pressure, and it must push back against recessed areas (lumbar spine) to maintain neutral alignment.
Soft (4-5/10)
High pressure relief. Allows deep sinkage.
Medium (6-7/10)
The industry standard. Balances yield and support.
Firm (8-9/10)
Maximum pushback. Minimal sinkage.
The Danger of Sag
A mattress that sags throws the spine out of alignment (creating a hammock effect), causing the muscles of the back to fire continuously throughout the night in a futile attempt to stabilize the spine. You wake up exhausted and in pain. Replace a mattress the moment a permanent depression exceeds 1.5 inches.