This is the best car in the garage.
We use articles like the and a before nouns, like car. The word "best" is an adjective, and adjectives do not take articles by themselves. Because the noun car is modified by the superlative adjective best, and because this makes the noun car definite in this context, we use the.
It is best not to do something.
Here, we have the adjective best, but this adjective is attached to no noun.
The adjective best is used in a copular construction with the dummy pronoun it. This pronoun does not refer to any object. There's no noun that we can attach the to here.
To answer your first question, yes, both sentences are correct. However, they do have different meanings.
The first sentence means what you think it means:
It is better to stay here than to go away.
Note that it could also mean this:
It is better to stay here than someplace else.
It really just depends upon context. The second sentence, as you said, contains a superlative, "best." In English, unlike in some other languages such as Spanish, the superlative does not require a definite article. In fact, it would actually sound weird to say, "It is the best to stay here."
The second sentence means this:
It is better to stay here than anywhere else.
That looks a lot like my second meaning for the first sentence, but here's the difference: "someplace else" in the first sentence refers to a specific other place, while "anywhere else" means any place where you could feasibly stay.
Best Answer
The sentence is grammatically and logically incorrect. Using two items and referring to them as one and singular is a contradiction. A couple examples of how to construct the sentence would be "Honda and Toyota are two of the best selling car makers in the US." or "Honda and Toyota are each one of the best selling car makers in the US."