Reverse curl or hammer curl?

Both train the brachialis. The hammer curl allows more load; the reverse curl adds more forearm extensor work. Doing both covers the arm well.

Straight bar or EZ bar?

An EZ bar is kinder on the wrists in this grip and lets most people keep the position honest.

A biceps curl with an overhand grip. The biceps loses leverage, so the brachialis and brachioradialis take over, which builds arm thickness and grip.

Primary: Brachialis, Forearms. Secondary: Biceps.

Hold a barbell, EZ bar or cable with an overhand grip at shoulder width.
Stand tall with the elbows at your sides and the wrists straight.
Curl the bar up without letting the elbows drift forward or the wrists collapse.
Lower under control to straight arms.

Wrists bending back under the load. Keep a straight line from knuckles to forearm; lighter loads make that possible.
Swinging the bar up with the shoulders. Stand against a wall for a set. If the bar stops moving, the load was the problem.

10 to 15 reps. Expect to use noticeably less weight than a normal curl; that is the leverage change, not weakness.