What is the fastest car in the world today?

I want to know the fastest car in the world (not including jet cars or anything like that). It could be street legal/non-street legal, also, it can be in production, a one-off model, or a concept soon to be put into production. Also, when saying your idea of the fastest car in the world, include some information about the car such as a top speed or how the idea of making the car was made or came into thought.