Independently: The form of grading is best because it removes any bias that a coin grader may have. A consensus grade is determined by a panel of numismatists. You should buy only rare coins which have been independently graded, authenticated, and certified by one of the top grading laboratories.

Subjectively: Many numismatic firms who sell rare coins also grade their own coins. This is clearly not a very objective method for coin grading because a seller might view a coin differently than a buyer because s/he has a vested interest in making a profit.