Hi sas95,
It’s difficult to say what would be the “best” topper for a mattress because the mattress underneath the topper would have a significant effect on how the topper feels.
Post #2 here and the topper guidelines it links to have some suggestions that would be helpful but if you are looking for a softer feel I would probably consider something in the range of upper teens to lower 20’s in terms of firmness and between 2" - 3" in terms of thickness (thicker will “feel” softer and would have more of the “feel” of latex and less of the feel of the mattress underneath it). If your mattress isn’t really firm then I would probably lean towards 2".
Phoenix