I’ve been giving Arthur the edge over Namor for some time, but it could a very narrow edge. It really depends on the battlefield. Namor does have the power to overcome him if he manages to trap him in a corner… big if, because Art is faster, and durable enough to turn a lot of Namor’s punches into glancing blows, and his aquatic telepathy gives him the ability to stun Namor, possibly even locking him stun lock as he gets off several clean hits whittling Namor’s endurance.
That’s without Mera’s powers.
Aquaman 6/10