What's the most comfortable mattress?

I bought a pillowtop mattress set a couple years ago and threw out my old one which was less than a year old bec it was just too hard but my new one is not as soft as I like either. I find myself sleeping on the couch 80% of the time. What's a really soft soft mattress that is comfortable?? The salesmen at mattress stores are no help in my experience. They're big guys who don't get what soft means, I can't sleep on anything that's hard. Makes me toss and turn and feel uncomfortable. help!