Greens:
Evelyn
Greg
Kingston
Mother North
Telluria
I had costume Lianna mentally queued up for next ascension, but then the Atomos buff throws me a curveball!
Some factors that make me consider Atomos:
I’m thinking that the self-resurrect is somewhat devalued when I’m running with Mother North? Not sure.
Eve, Greg and Kings are not pure sniper compared to Lianna.
Atomos is the hit all attacker, but too weak to get rid of a particular opponent hero in a fight.
I have both Lianna and Atomos, although, I use them both in the offensive line up.
If I really have to choose either one, my priority would be toward Lianna.
Atomos is more useful on the defense as a tank. (Could be viable if your alliance runs green tanks BUT in that case, you’d use Telluria anyway)
He could be useful on offense - but definitely no where near enough to prioritize him over Lianna with costume bonus - the hardest hitter in the entire game, by some margin.
It’s okay. You don’t need one.
You have Evelyn + 3 green snipers; you’d be pretty much taking out 3 opponent heroes every time you unleash specials (2 opponents at the very very least).
I don’t see much use of an AoE hero in that green stack.
Hope this helps.
Cheers!!
In my honest opinion, I would focus on Lianna with Costume first - 100%
Whichever hero you choose good luck
I recently maxed Atamos because of the Eve then AoE factor. I’m happy with him. He works well with MN as she can either boost him from 30% health or resurrect him. His damage on enemies with the elemental def down is usually between 250-500. I also chose to max him because of my Atlantis team roster.
Overall he’s ok.
I agree with @CaptainjaKCsparrow. Using Kingston, Greg and C-Lianna you don’t need him to cause group damage. I would probably do her first.
Another vote for Lianna. 
I guess we need a new ridiculous standard now: Never thought I would ask this; Boomer or Lianna!
Lianna BTW … for both. 
Good luck!