These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
YMMV: Black Widow
Breakout Character: In the MCU at least; she showed up in Iron Man 2 but was something of a Base Breaker at the time. The Avengers, however, gave her a huge boost in popularity and a lot of depth, along with giving future films a lot more to work with. Now she's the most prominent character without a film, and even that is likely to change soon.
Fan-Preferred Couple: By default of no longer being together, her and Bucky became this for comic audiences. With the films, her and Clint is also the most popular het couple, but even there Bucktasha is still popular there.