Luna has done very well as Robbie Reyes, and the writers have used him well, I think. That scene in the last episode where he walks through the prison riot and the prisoners are rushing to get back in their cells was too good.
I've enjoyed Brandon Routh as Atom as well. Though it may not be the most precise characterization to the comic, he brings the right notes of optimism, adventuring spirit, and ingenuity that makes Ray Palmer great.