There have actually been five
James T. Kirk, played by William Shatner (Star Trek, the Original Series)
Jean-Luc Picard, played by Patrick Stewart (Star Trek: The Next Generation)
Benjamin Sisko, played by Avery Brooks (Star Trek: Deep Space Nine)
Kathryn Janeway, played by Kate Mulgrew (Star Trek: Voyager)
Jonathan Archer, played by Scott Bakula (Enterprise)
Of course, there have been many more Captains seen on Star Trek, but I assume these are the ones you mean as they were considered the lead actors in each of their respective series. Hope that helps!