I just put instant coffee in my cup and pour the water. My question is, what temperature should that water be?
You have quite a bit of leeway with Instant Coffee. You can brew it using water of your desired temperature. I would recommend using water just below boiling point though, around 95-99 degrees Celsius. You could use 85-95 degrees Celsius water as well; it won't make a big difference in terms of extraction of flavour, and here is why:
How instant coffee is prepared in the factory:
The result from this is what you get in the store.
All of the very particular factors that one must consider when trying to make great coffee have already been decided for you. You have no control over roast, grind, time from grind to extraction, extraction time and temperature, or time from extraction to consumption. All of this been decided for you.
It is up to you. Brew it at whatever temperature you like to drink your coffee at.
However, it may be slightly harder to dissolve the coffee and get good flavor at cold temperatures. Keeping it on the hotter side may be better. That's just chemistry though.
See the National Coffee Association's pretty comprehensive guide to preparing coffee. Assuming you do not mean instant coffee, they recommend a temperature of 195 - 205 F.
You may know that you can buy special kettles that heat the water to 92 deg c to avoid scalding the coffee. These are expensive and unnecessary. Here's an idea that I came up with.