These two are some of Marvel’s toughest. They have defeated many opponents and will keep winning. However in the comics and shows Ultron has shown that he can take down pretty much anyone. He possibly can’t be defeated by an Avenger. Ultron wins.
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Generally it takes several Avengers to kill Ultron. The best Thor can do is delay the inevitable.
Agreed, one Avenger cannot defeat Ultron on his own.