I think the answer is:
Judith Olivia Dench, the British actor portraying M in many James Bond movies.
The actors share a common thing:
They have all played roles with one letter names. I realized after finishing the list that the title is a clue here, The A-list:
Here's the list:
1. Chris Hemsworth - Agent H in Men in Black: International
2. Tommy Lee Jones - Agent K in Men in Black Judith Olivia Dench - M in many James Bond movies
3. Ralph Fiennes - M, in No Time to Die
4. Emma Thompson - Agent O in Men in Black: International
5. John Cleese - Q in Die Another Day
6. Nicholas Hoult - R in Warm Bodies
7. Henry Cavill - Superman (His logo is just the S)
8. Liam Neeson - Agent High T in Men in Black: International
9. Hugo Weaving - V in V for Vendetta
10. James McAvoy - Private James W. Miller in Band of Brothers Professor X in X-men First Class
11. Patrich Stewart - Professor X in X-men Days of Future Past
12. Anotonio Banderas - Z in The Mask for Zorro
There are several options for the missing one:
The list is in alphabetical order, so the options are any letter between H and M, i, j, k, l. Apparently there can be duplicates, so H and M are included.
My guess is that you're after
Agent K in Men in Black, since there are other Men in Black actors, but I could be wrong.
Judith Olivia Dench, the British actor portraying M in many James Bond movies.