Hmmm. Black Sky has a good deal of superhuman strength, but not as much as Red Guardian. On the other hand, she is insanely skilled in addition to her strength. The issue in Widow was that Natasha had all the skill, but Alexei had all the strength. Neither of them had that combo to take against Taskmaster. But that was in the suit. Without the suit, I give it to Elektra. She's stronger, skilled, and ruthless.