In short, the best is one that you lay down on and feel comfortable with. But weight, and the size that it rolls up to might be a deciding factor depending on how your going to use the swag.
I don't go hiking or anything with mine, so in addition to the soft foam one that came with it, I bought a thiner, stiffer, sponge mattress from Clark rubber. It rolls up ok - not great, and it adds a bit of weight but its comfortable, . Go in to a camping store, or clark rubber, or anywhere you can find them, roll em out, and try it right there in store
To give you an idea, and maybe a starting point, I have one softer mattress, about 2" thick, and the other is a fair bit firmer but 1 1/2" thick. Its pretty comfortable on the ground, and even better when I'm on my roof rack, as it gives it a bit of springiness
Other option is a self inflating one. I've never used one but they look the goods if you buy a quality one, just don't get something that needs inflating as it inevitably springs a leak