They are all at 3/70 and I have a fully ascended Gravemaker.
I am leaning toward Marjana over Zimkitha.
It is because Marjana’s special skill has a greater increase of attack, and Zimkitha’s special skills of the elemental link and dispel are not tied to her power levels.
I agree (based on my limited knowledge of your roster): Marjana.
Khagan and Zimkitha are most valuable for their specials, nit their attacks. Specials are maxed at 3/70.
Marjana and Natalya both improve (greatly) with their fourth ascension. Marjana is a solid sniper. Natalya may fo more damage, but its over time, which gives the target opportunities to attack or cast against you. Mana control is nice, but dead heroes cast no specials.
Id say Zimkitha. With her and Gravemaker you would have enough power to peel off foe’s HP AoE and finish them with tiles or another round of specials.
You are against Hel? Zimkitha!
You are against Yunan? Zimkitha!
You are against Zeline? Zimkitha!
You are against Gravemaker? Zimkitha!
Her special would stay the same for level 3.70 and 4.80 but… a 3.70 Zimkitha is dead pretty quickly and at 4.80 she would be way more harder to deal with with her increased stats and self recovery.