It took me a while to come up with my answer, but I'm leaning towards Green Arrow. In the Marvel movies, we really haven't seen BW do anything truly noteworthy, other than take out a few generic guards in Iron Man 2, beat a brainwashed Hawkeye, and kill a couple of Chitauri soldiers (Who, lets be honest, didn't look like the most skilled fighters). However in Arrow, Ollie is constantly up against skilled martial artists and marksmen who push him to his limits, and from these encounters we have seen just how skilled his experience on the island made him.
Another area where he has the advantage is the setting of the battle. 50 feet apart, with cover and at night. We can assume BW will be using handguns and from that distance, and she's going to need to get much closer if she's to have any chance of hitting him. GA on the other hand will have an array of arrows that he has shown great skill in using from distance, if he fails to hit her with his initial shots, he could always turn to an explosive arrow to provide cover for him to stealthily traverse the street to get a better shot. In the episode with Deadshot's first appearance, all he needed was a bit of cover and one clean shot to "kill" him. Imagine what he could do with 50 feet between him and his enemy, and at night, where GA primarily works. GA takes the battle.
And for anyone who brings up the Hawkeye vs Black Widow fight from The Avengers, remember, Clint was holding back, and the fight took place in close quarters where he couldn't get a proper shot off, it's hardly a fair comparison.