Black Widow. (also, watch this space, someone asked me to talk about my ideal CA movie, so I'm going to be doing that.)

If the Winter Soldier has an excellent Natasha arc and continues the awesomeness that was Natasha in Avengers, the rest of the movie could be a dumpster fire and I would barely care. If everything else is amazing but Natasha is sidelined, the movie is dead to me.