Nicholas Courtney (the Brig) was in more episodes of Doctor Who (101 episodes) than Colin Baker (the sixth Doctor, 34 episodes) and Sylvester McCoy (the seventh Doctor, 44 episodes) combined. He was also in more than Peter Davison (the fifth Doctor, 76 episodes)
and of course, he's been in more episodes than Paul McGann (the eighth Doctor, 0 episodes) but that's cheating because he was in the TV movie, the Night of the Doctor minisode, and like a trillion Big Finish stories
the first, second, third, and fourth Doctors were all in more episodes than the Brig