I don't know about the Black Widow. She's a third tier character to me. It'd be hard to come up with a favorite moment.
As for Quicksilver, I had a good chuckle from the Avengers/JLA limited series when he out performed the Flash at one point and yelled, "Speed-Force be damned! I'm faster than he is! Ha! Faster!" That was so Pietro.
The Scarlet Witch is a lot harder. There are many great moments for her. It was impressive when, after a team of about 8 Avengers couldn't even scratch Ultron, she single-handedly destroyed him. Avengers #171 I think.