Well, the temperature that maximizes biodiversity across the planet.
Could you expand on why "biodiversity" ought to be the goal? If I had to pick something, I'd have picked "comfort of humans" or, perhaps, the humans' longevity or something like that.
Why do you pick "biodiversity"?
Maximizing biodiversity is a decent goal to have high on your list. The more organisms there are, the more resistant a given system is likely to be. If you've got one species of tree in a forest and beetles come and wipe out that species, you're in trouble. If you've got high biodiversity, you're more likely to have less trees that will be affected, plus a better chance that there's somebody that calls the beetle dinner.
Why should humans care about resilience? We derive a lot of services from natural systems. Protection from extreme events (flood, fire, insects, etc); diverse food stocks; tourism; unique chemicals for pharmaceuticals; groundwater purification; local weather stabilization; and so on. Even if you don't "like" nature, you derive a tremendous number of services from it. The best way to maintain longterm comfort/longevity of humans is to make sure those systems continue to be able to perform those services.