Steel bending in conjunction with big compound movements. You get a lot of time under tension. Sledgehammer levering is also good. Bicep curls are great too espscially with a thick bar.
I was bestowed with larger forearms,and tiny biceps, deads seem to tax my forearms the most rows and chins should be a staple hammer curls feel great as well
like some of the people already said, rows and deadlifts will probably do the best. along with chin ups/pull ups. fat grips are probably the only other thing id be interested in. but heavy rows and a lot of them definitely help.