This is a question I hear all the time. Ideally you want a mattress that is going to keep your spine aligned. Things to consider are your width and weight.
Generally wider people are going to need a softer mattress to compensate for their curves. Thinner people can use a firmer mattress, generally, because they don’t have as many curves to consider.
Please take your weight into consideration. If you are heavy and wide, you may need something medium soft, just so you don’t sink too far into the mattress.
Your best bet is to go to the mattress store, and spend ten minutes or so on each mattress. If the mattress is right for you, you should start to feel your back and hip muscles relaxing after about a minute.
Remember, you spend approximately 1/3 of your life sleeping. Considering this, a mattress should be a large priority for people. Don’t be afraid to spend a few grand on a good mattress. It will improve your sleep, support proper sleeping posture, ultimately leading you to feel more rested and comfortable.