Start with the 2003 miniseries
Watch the main series Battlestar Galactica 2004–2009
Watch Battlestar Galactica: Razor after Season 3
Watch Battlestar Galactica: The Face of the Enemy between Seasons 4 and 4.5
Watch Battlestar Galactica: The Plan after the main series
Watch Caprica if you want the prequel series
Watch Battlestar Galactica: Blood and Chrome if you want the later prequel story
Watch in release order for the simplest viewing experience