Ultron can be considered among one of the Avengers' strongest adversaries while, alternatively, both Hulk and She-Hulk can be considered the strongest members of the team. It wasn't until the Age of Ultron story arc that audiences had the chance to see which side sprouted the strongest fighter. Bruce Banner himself is largely absent from the storyline, but it is mentioned in passing in Age of Ultron #3 by Brian Michael Bendis and Bryan Hitch that he was killed off-panel by Ultron.
She-Hulk, although she was not killed off-panel, was dispatched just as unceremoniously. In Age of Ultron #4 by Bendis and Hitch, She-Hulk is fighting off Ultron drones alongside Luke Cage when suddenly, out of nowhere, she's blasted in the head by one of the drones. Interestingly, Jennifer Walters' death came moments after she was boasting about being "almost the strongest one there is."
No one ever could deny just how powerful Ultron is, but the A.I. in his final form proved just how strong he is when he took out the poster children of Gamma Radiation. Even more impressive, he did so without either posing much of any challenge to Ultron. The death of both Hulks is treated with little to no fanfare, not as a disrespect to either character, but to showcase the sheer amount of inconceivable power that Ultron wields at his fingertips. A fight with Hulk alone feasibly could have made for a climactic battle, but it didn't because Hulk is no match for Ultron. That's likely why he's taken out so early on in the story, before the events of the first issue. Readers aren't even made privy to the exact circumstances of Bruce's death. Hulk's demise ultimately amounts to nothing more than another body on Ultron's count.
Meanwhile, She-Hulk, who acquired her powers from her cousin's blood transfusion in Savage She-Hulk #1 by Stan Lee and John Buscema, is more or less just as strong as Bruce Banner, with the latter likely being the strongest due to taking a direct hit of Gamma Radiation instead. This suggests that it more or less took the same amount of effort to kill Hulk as he did She-Hulk.
The technology of Ultron is so powerful that two of the most dangerous Avengers can get insta-killed casually without either seeing it coming. Ultron defeated both She-Hulk and the Hulk not by outsmarting them or even besting them in a real one-on-one fight, but by overwhelming them both with his strength.