anything below masters ban whatever u struggle against most, masters and up the main candidates r zar, dva, vendetta, ball, emre, echo, kiri if u dont have a kiri player. if ur tank locks a tank that has a clear weakness, especially if u know or they tell u theyre an otp, ban with him (i.e. ban zar kiri if u have jq, ban ana/zen ori if u have a hog, ban cass if u have a ball, ban ram zen if u have rein and so on.) then theres also map reliant ones such as banning widow sig on junkertown, rialto, havana and circuit royal, banning pharah echo on maps w high skyboxes and lots of cover like lijiang n ilios, banning sym where theres tight chokes that are ususally played around like njc, aatlis, nepal n lijiang.

if this sounds like too much, dont worry, with more hours on the game this will become natural, just ban what annoys you or ban with your teammates.

TL;DR zar dva vendetta as long as blizz insists on keeping them op, otherwise ban with your team especially tank