For years, I have always focused on forearms, but through time have found that they respond with a more subtle, indirect approach.
What does that mean?
Pull-ups, both the heavy, weighted variety and also thousands of reps through time with just body weight.
Another would be dips. You will absolutely not ever hear of dips as a forearm-builder, but again with a marriage of heavy loads and moderate body weight, volume-focused work, you will see great results.
Finally, I want to pay homage to a very primal and humbling movement.—-The Farmers Walk.
Grab a couple heavy dumbbells, kettles, implements, whatever and walk until you drop them.
This will build your forearms better than any direct wrist curl or reverse curl in my opinion.