What is a good computer game

Well, it really depends on what you like and prefer. There really is no simple answer as there are a multitude of genres and platforms so there's something that suits almost everybody out there.

If the purpose of the question is to find a new game to play, I suggest reading reviews on gaming sites, try demos of new games and ask friends what they are playing and enjoying.

If you want to find out what games are considered "the best", a more specific question would be needed as to what platform, genre, year or decade you would be interested in.