Depends who you ask. This is a contentious question.
Everyone agrees that the television Master was played by Roger Delgado, Peter Pratt, Geoffrey Beevers, Anthony Ainley, Eric Roberts, Derek Jacobi and John Simm. (In fact the last three still cause arguments in some parts of fandom.)
In Series 8, Michelle Gomez played Missy, who was was eventually revealed (in "Dark Water") to be a female incarnation of the Master.
Gordon Tipple appeared very briefly in an unspeaking role in "The TV Movie", but was still credited as "The Old Master".
The FASA Role Playing Game, Target novelisations and a 1988 episode of Mastermind all explicitly state that The Monk is the Master. In fact, Peter Butterworth is listed among the actors to have played the Master in the FASA Game Module 'Master'.
Jonathan Pryce played The Master in "The Curse of Fatal Death". However, this story's status in the continuity is far from agreed upon.
Big Finish Audio have cast actor Alex MacQueen as the Master in original Audio Dramas. However, again, some people question the continuity status of Big Finish Audios.
According to several sources such as the Target novelisations and real-world interviews the War Chief is a prior incarnation of the Master. This would add Edward Brayshaw to the list.
Finally, some people would add David Garfield to the list. The Big Finish Audio "The Hollows of Time" (audio story) was an adaptation of an unmade television story. In it, the villain is a disguised Time Lord calling himself 'Stream'. 'Stream' is clearly an anagram of 'Master'. In the television version, Stream would have been revealed to be the Master. However, in the audio version, this revelation is never explicitly made.
Additionally, Emrys Jones played the Master of the Land of Fiction in 'The Mind Robber', who is definitely not the Time Lord the Master, but he was credited as simply 'The Master'.