Our best suggestion for any of our clients always takes into consideration both your environment, and how you use your hot tub.
If you are in a climate that has cold or very cold weather, and you keep the water very warm or hot, then the best cover to keep your operating costs low would be the thickest cover you can find. In today's market, that means a cover that is 6 inches thick. The general rule is, the thicker the cover, the more it will insulate.
In keeping with that philosophy, we would then recommend upgrading your cover with a full hinge seal. The full hinge seal is an additional piece of foam added to the bottom of the cover, that blocks heat from escaping through the center (fold or hinge) area of the cover. A full hinge seal will add R-9 to any hot tub cover.
The next significant factor is whether or not the cover will need extra strength to hold up any significant weight such as snow, pets or children. Most standard hot tub covers are manufactured with 1.0 pound density foam. Upgrades are offered to either 1.5# or 2.0# density which increase the foam density by 50% and 100% respectively.
For a base understanding of cover strength, using a standard four inch thick cover tapering to two inches thick at the edges, you can view their respective strengths below.
Standard 1 pound - Shear strength 18-22 psi (Estimated static load limit - 190 pounds)
Deluxe 1.5 pound - Shear strength 26-32 psi (Estimated static load limit - 270 pounds)
Ultra Deluxe 2 pound - Shear strength 33-37 psi (Estimated static load limit - 370 pounds)
Keep in mind, if the foam core of the cover ever breaks, the cover warranty will be voided, as foam core breakage is considered abuse in the eyes of the cover manufacturers.
If you are worried about your cover strength, consider that thicker covers are stronger than thinner covers so if you need a thicker cover to get the maximum insulation, you are already gaining more strength than shown above. This may allow you to upgrade to only the 1.5# density foam to get the strength you need for your environment.