there is no "best" build for a adc, its all situational.
items you should always get are: berserkers,lw,ie
rest is situantional, for example botrk provides more utility and faster stacks for tanky enemys. bloodthirster best ad source and can be taken for example squishy enemys without much assasins. ga and banshees is also different, regular ga is better but if you have to block gamechanging spells you should take it