The best solar cover is one you will actually use. The thicker the cover, the heavier it is and the less likely you will use it. I have not found a significant difference in cover life with the thicker covers so my preference now is to go with a lighter cover. Current I am using a 5 mil cover and it is very easy to take on and off.
As for color, I haven't found much of a difference in heat gain although I have only tried blue and clear. Theoretically, clear will allow more sun light directly into the pool (and radiation out of the pool) but that doesn't mean water temperature will be higher. The colored covers heat by conduction and re-radiation and seem to allow heating of the pool fairly effectively as well.
The biggest factor in a solar cover is simply covering the pool. Everything else is secondary.