This is another What if scenario. During the Red Hulk's rampage, there was a moment that Ares got into a bit of a disagreement with Namor, telling Namor that if he continued to voice his opinion that he would have curbed him. Now What If there were no heroes there to hold either of them back? Who really would have won that battle?
Osborn in his Iron Patriot backed also off, when it came to a possible fight with Ares. Osborn had the strenght advantage I guess (100 tons, I would say), the range and the flight advantage, yet he was smart enough to skip it.
Ares would defeat Namor, especially if he wields weapons.
Namor fought Ares in the old Sub-Mariner comic. They were underwater and Ares attempted to change shape into sharks, squids etc. but couldn't get the upper hand. Ares tossed him away then Namor created a water vortex around him, trapping him. But after that Ares had his strength drained by Venus, and Namor was able to toss him around.
They also fought in the Avengers Olympus storyline around #280-285. Cap was busted up and gave Namor his shield, and Namor broke Ares' axe with it. But Namor had been away from water too long to fight at full strength. Captain Marvel pushed Ares into a pool of water, at which point Namor jumped in and beat the crap out of him. Later Namor was seen dumping the unconscious Ares on the ground.