The best ratio is zero atk and 100% cdmg ;)  to a point, of course, but I don't think you'd be at that point.

Def based is similar, although you benefit twice from the def, so you can afford to  shift a bit more into that.

The breakpoint for Atk/Def versus CD is different for each toon based on their stats, and as stats change, so does the breakpoint. This post on the HH website dives deep into this topic and gives you a formula that can be used.

https://hellhades.com/how-your-damage-is-actually-calculated-in-raid-shadow-legends/

That is a confusing (and factually incorrect) answer from someone typically quite helpful. 

Y'know, I was going to argue, but decided to do the math, and you're right, my answer wasn't really accurate. Was looking more in context with the numbers I'm used to seeing.

Assuming a standard lego champ of base ATK 1500,

A / 1500 = 1 + (C / 100)

A = 3000, C = 100
A = 4500, C = 200
A = 6000, C = 300

As an example, at ATK = 6000,

6090 * 3.5 = 21315, 6090 * 4 = 24360, 6090 * 4.5 = 27405
6000 * 3.56 = 21360, 6000 * 4.06 = 24360, 6000 * 4.56 = 27360

So, up until 300% CDMG, you're better adding more CDMG than ATK. But at lower ATK numbers, the ATK% does win out much earlier.