The last two Black Widow series have been absolutely incredible, and I can't recommend them enough. Both are filled with espionage and action, and both are at the top of the most beautiful art I've ever seen in comics. They make great starting points for new readers, and the series are completely unrelated to one another, so just pick whichever looks more interesting to you, or read them one after the other.
Every book of the 2004 series, in reading order:
Every book of the 2016 series, in reading order: