Info from IMDB.com

170 Episodes

• Meredith Grey
• Christina Yang
• Alex Karev
• Miranda Bailey
• Richard Webber
• Derek Shepherd

142 Episodes

• Callie Torres

134 Episodes

• Mark Sloan

118 Episodes

• Izzie Stevens

111 Episodes

• Lexie Grey

100 Episodes

• George O'Malley

91 Episodes

• Owen Hunt

82 Episodes

• Arizona Robbins

65 Episodes

• Jackson Avery

61 Episodes

• Preston Burke
• Kim raver
  

59 Episodes

• Addison Montgomery-Shepherd

57 Episodes

• April Kepner
  

36 Episodes

• Joe

35 Episodes

• Steve Mostow
  

28 Episodes

• Nurse Tyler Christian

26 Episodes

• Intern Megan

24 Episodes

• Intern Graciella
  

23 Episodes

• Erica Hahn

17 Episodes

• Jane Doe/Ava/Rebbecca
• Nurse Olivia Harper

15 Episodes

• Ellis Grey

14 Episodes

• Denny Duquette

13 Episodes

• Nurse Linda

12 Episodes

• Ray the Paramedic

11 Episodes

• Patricia

10 Episodes

• Thatcher Grey
• Adele Webber
• Nurse Rose
• Anesthesiologist
• ER Resident

9 Episodes

• Dr. Finn Dandridge
• Nurse Kate

8 Episodes

• Dr. Sydney Herron
• Intern Pierce

7 Episodes

• Tucker Jones
• Intern Laura
• Intern Steve
• Harold O'Malley
• Intern Lucy
• Lab Tech

6 Episodes

• Susan Grey
• Nurse Ginger
• Nurse Debbie
• Nurse Vivian
• Jill the Paramedic
• Dr. Raj Sen

5 Episodes

• Louise O'Malley
• Ronney O'Malley
• E.R. Nurse
• Intern Dani
• Jane Burke
• John the Paramedic
• Intern Claire
• Anesthesiologist

4 Episodes

• Walter
• Scrub Nurse
• Lab Tech
• Intern Mitch
• Nicole the Paramedic
• Intern Leo
• Dylan Young

And many more minor cast stars who appear in three episodes or less.