"on who do you rely the most to make damage"

One answer would be 'inspire' tanks like Violette. If you mean the tank shooting, then any tank with a significant amount of range is good since you're devoting all/most emblem space to attack. If you mean what damage dealing tank is the most critical I'd say any tank that can knock out enemy damage dealers.

Victoria/Catriona - Can fire multiple rounds with perfect accuracy while flanking or just maintaining range. If you absolutely, positively can not afford to crew around missing shots.
Scarlett[+] - Tiny range means she precisely cuts through distractions to stick a pitchfork through what ails you. Does a semi respectable job of soaking up damage as well.