...the "Princess and the Pea".
We went with 100% latex - hygienic (no bedbugs!), it 'breathes' and it's long lasting but expensive. We both love it! It's a California King Bassett Carrington Chase (made by Sealy & they make the Stearns and Foster brand as well) Bedford Knoll I believe is the particular model. Stay away from pillowtop mattresses even if you like a soft top - pillowtops give away (flat, lumpy) easily after a couple of years. Ours is a 'plush tight top' - which is firm underneath but has a plush top sewn onto the mattress. You can always get a soft mattress pad for the top (Amazon) if you like it even softer. If you look at latex, be careful not to confuse the Sealy Comfort Series mattresses that are latex - they aren't 100% latex and they don't last as long. It's the Sealy Spring Free that are 100% latex.
I hope this helps! It's so hard to commit to something that is so expensive and that you expect to last a long time.
For anyone reading this thread, if you have a pullout sofa and need a mattress for it, the best $300 I ever spent was on an 'Air Dream' air over coil mattress - that thing is fantastic. So comfortable and we've had it for about 4 years and have had zero problems.
I have a good friend who loves her tempurpedic mattress - but I don't like it that much at all... holds body heat and doesn't breathe and my husband is a 'furnace', so that was out. Plus they are $$$ for a mid range model.