Sleep comfort is highly subjective. Some people like a super soft mattress they can fall into, whereas others find rock hard comfortable. From what I can tell, firmer is generally better for your back, though it can take time to get used to. So firm for back/stomach sleepers, and medium-firm for side sleepers. But again, it ultimately depends on you.
Over the years I've learned that I *strongly* prefer a natural latex mattress with support coils on a slat base for airflow. The latex just breaths a lot better and dissipates excess heat and moisture, which means I don't wake up in the middle of the night drenched in sweat. A cotton+wool comforter also helps with this. Latex doesn't off-gas VOCs, and it's biodegradable. However, it is more expensive. I expect to get 10+ years from a mattress so I don't mind paying a bit more, especially if that means it won't be in a landfill 10,000 years from now.
Our most recent purchase was from Avocado Mattress and we're very happy with it. Though I'm sure there are other natural latex mattress companies that make quality products.