It is very important with the thousands of choices in the mattress world, you make sure you choose the best mattress for you. Your mattress must support you meaning have the ability to keep your spine aligned in all your sleeping positions. You are looking to see if your spine is straight when you are on your side or follows its natural "S" curve when you are on your back or stomach. When you have 2 - 3 mattresses that seem to provide the best support and alignment, then its time to go home and compare the mattresses to see any common patterns in the ones that worked best for you.
Make sure your mattress is made for you!
If any "parts" of you sink in too much (usually pelvic area) or too little (usually shoulder area) then you will sleep out of alignment which usually leads to pain or discomfort in the lower back, upper back, or neck and shoulders (which can also be from the wrong pillow for your sleeping positions). It also helps to "stand up straight with good posture" before you lie down on your back or stomach so they can "memorize" what your natural "S" curve looks like and see if it looks the same when you are lying down. You should also make sure there are no "recessed areas" in all sleeping positions to make sure that they are being "filled in" and supported. If it is too easy to slide a hand under then it means that the mattress support layer could be too firm to allow for good support and alignment. If you are "hammocking" and the heavier parts are sinking down too far then this too can means poor support and alignment. Pay particular attention here to make sure that the pelvic area isn't sinking down too far, that the shoulders are sinking in far enough, and that the head and neck are in alignment (always test a mattres using a suitable pillow).
The best mattress for your is all about personal preference and you bodies physical needs. You could choose ultra-firm (hopefully not) and ultra-plush, it is all about making sure you have the back support with the right combination of padding to make you feel comfortable.
From my experience in the variety of Portland mattresses (mattress world), the more curvy you are the more padding needed to fill the arch in your back and the behind your neck as well as the spaces behind different parts of your legs.Back sleeping places most of your body weight on your upper back, bottom, and heels. A bed that is too firm and rigid can cause pressure points and leave your lower back unsupported, contributing to muscle tension and pain. A bed that is too soft can also lack support or allow your upper body to sink too far in, also contributing to discomfort. Your ideal mattress is likely in the medium to medium-firm range, with a comfort layer just thick enough to cushion pressure points, but not so thick or plush that your upper body sinks too deep. You should not be able to fit more than 1 finger between your lower back and the mattress when lying flat.