Comfort does not come from one “best” foam; it depends on seat dimensions, filling, support and your habits.
Consider the whole seat construction
The sitting feel comes from the support system, the properties of foam or other filling, the combination of layers and the upholstery. The same foam can feel different at another thickness or when combined with a different material. A filling name alone is therefore not enough to predict the finished sofa’s comfort.
If technical details are available, ask about the material type, density and intended function of each layer. Density and perceived firmness are not the same measure and should not be confused.
Find dimensions that suit your body
Seat height, depth and back position affect how easily you sit down and stand up, as well as whether your feet rest comfortably. A deep seat can be relaxing, but conversation or a shorter user may call for an extra cushion.
If more than one person will use the furniture, try it in different sitting positions. Sit down, lean back, stand up and spend more than a few seconds there. Comfort is personal, so trying the furniture tells you more than guessing from a photograph.
Comfort also changes with use
The initial feel of a new seat may differ from the feel you get used to over time. Arrange loose cushions according to the maker’s instructions, and check the warranty before attempting to open them or change the filling.
Look for information about the filling and care for the specific model. A general claim that one material is always softer, more durable or better for everyone cannot replace trying the furniture and reviewing its full specification.