ADC is a mechanically intense role by nature. A couple champs work around that to an extent, such as Jhin (since his attack speed is basically fixed), Ziggs (since he doesn't build any), and Ezreal (since he does a significant of his portions with Q, similar to Ziggs). But for Varus, Twitch, and Kog, you're going to have to learn how to kite while autoattacking, and doing that effectively requires a fair amount of micro skill. Those three also aren't particularly safe as ADC, since none have a dash or hard CC outside of an ultimate cooldown.
Safety and high damage output are generally at odds on ADCs. I mean, they all do great damage over time compared to other classes. But the safer ADCs have less damage, and the higher damage ADCs have less safety. Tristana has higher-than-average attack range, a long range dash, and an AoE knockback, but her damage suffers for it. Kog, Vayne, and Twitch don't have reliable hard CC or escapes, so they get higher damage in compensation.
So if mechanical simplicity is your top priority, I'd say Jhin, Ziggs, and Ezreal. If you want to be safe in team fights, a champ based on abilities (like those three and Varus) or someone with a dash and hard CC, such as Caitlyn or Tristana. And if you want tons of damage, Twitch, Vayne, or Kog'Maw.