Well, technically there is no "best" tire for a certain condition, it really depends on what class you are racing in. For example, on a shifter kart, you would want to have pretty much the most grippy tires you can get, because you need it.

But in a class that only has 10hp for example, you would definately not want the most grippy tires because it will slow you down due to too much grip. If your asking what brand is best for tires, I would say that bridgestone and vega are the best. Vega's most grippy tires are very good, and are very grippy for a while, but on the other hand, the bridgestones less grippy tires are very good such as the yhc. The yhc's and the vega whites are both great tires for their application. But as TJ said, tires are almost ALWAYS spec so its not like you have a choice, if your racing at least.