After some research, I found that a pool solar cover between 12millimeters and 16millimeters was the best for heat retention. Thicker solar covers absorbed and retain more heat and are more durable against sun damage, chemical resistance, and ripping. Thicker solar covers aren’t as easily blown off the pool by strong winds.
Solar covers are typically between 8 millimeters to 16 millimeters thick. They are made to allow sunlight through but thick enough to be durable. Thicker covers are heavier and less likely to get blown away but are also more difficult to maneuver on and off the pool.
The thinner solar cover will allow more heat to pass through to the water, but a thicker cover will absorb more heat. So one needs to find a happy medium between heating the water and durability. I would personally buy a 16-millimeter thick solar cover as I will get value for money if I look after it.