Answer:
The most famous is the engraving made by Martin Droushout for the front page of the First Folio. Shakespeare's friends claimed that it was a good likeness. Also there is a statue in the church at Stratford which his family thought was a good likeness. Another picture that many people think is a genuine representation of Shakespeare is called the Chandos portrait. Recently a man called Stanley Wells has claimed that a picture called the Cobbe portrait is really of Shakespeare, but he is pretty much alone in that opinion, and in any event it doesn't look anything like the other ones. Another picture called the Grafton portrait is thought by some to represent a younger Shakespeare, and is helped by a label in 16th century ink on 16th century paper identifying it as Shakespeare, but many are still skeptical.
Best to stick with the Droushout.