Which is more effective voltaren (diclofenac) or naproxen for neck pain?

Both: Both medications can be effective for neck pain. Trying each one separately for a few days would better help you chose which one to take.

Pain killers: Naproxen and voltaren (diclofenac) are non steroidal anti-inflammatory medications (nsaids). They are both used to treat pain. The best pain killer for the pain is the one that relieves your pain.