It depends what you're looking for. The chess program included on Mac is generally stronger than most players, so if you're looking for a game just head to your applictaions folder. For analysis take a look at some of these programs.
http://www.sigmachess.com/_chesslinks/chesslinks.html#UCI
Many of those will need a chess interface to be played in, and that site (Sigma Chess) makes as good as a product as you'll ever need.
Hi, I just bought a new MacBook Pro & I would like to know what would be the strongest chess program for it? Thanks, Steve