Jason Isaacs by a country mile. Anthony Rapp, Doug Jones, Mary Wiseman, and Wilson Cruz are all solid actors/actresses. Shazad Latif has shown a huge improvement in the last two episodes with his PTSD and realization that he was more than just a prisoner to the Klingons.
That leaves Sonequa Martin-Green. I'm not sure whether it's her acting, the writing, or how the directors are suggesting she play the character, but we're 10 episodes in and I really don't care about Burnam. She and Latif have zero chemistry IMHO. She and Wiseman work well together, but I'm thinking that's more a credit to Wiseman and the fact that I find her character intersting.
** EDITED **
I forgot to mention Mary Chieffo, who also has been doing a great job and has made L'Rell a very interesting character.