This is tough because three of the greatest players in baseball history spent spent one third to one half of their career as members of the Boston Red Sox. I am, of course, speaking of Cy Young (192 Red Sox wins), Tris Speaker and Manny Ramirez.
My Mount Rushmore
1) Ted Williams
2) Carl Yastrzemski
3) Roger Clemens (192 pre-steroid wins – of his 354 total wins) as a member of the Red Sox
4) Jim Rice or Dwight Evans
(roughly equal career values, but Rice shows up better in the triple crown categories)