Yo... I've never seen a good Ultron Cosplay, that would be complete and utter pwnage!!!!!! Well, I'd want him to have a Vibranium Shell somehow, to account for his uberness. Then, he'd have that super hacking ability, and crazy intelligence so he could break down the Avengers psychologically, and loudly blast their personal info, or upset their lives in a way that is carefully calculated to bring about a certain result. He could turn the Avengers against each other pretty thoroughly and easily, I think. He could turn the whole military machine against SHIELD, in fact, which could be pretty cool. Beyond that, he doesn't need/probably shouldn't have a lot of physical abilities. A ranged energy attack, super strength and durability of course. His robot flexibility, speed and timing is what should allow him to face all the Avengers single handedly. Keep it simple, and keep driving home his intelligence and access to the Avengers' personal lives. He's not a villain with a lot of pathos, so there's no need to focus on his character arc or anything.