One must - read flash story could be 'The Gift of the Magi' by O. Henry. It's a short but powerful tale about a young couple's selfless love. Another is 'A Very Old Man with Enormous Wings' by Gabriel García Márquez, which is full of magical realism elements. And 'Cat Person' by Kristen Roupenian is also popular as it delves into modern - day relationships in a very relatable way.
Well, in 'best of flash fiction', often the stories that stand out are the ones with vivid imagery. Since flash fiction has very limited space, the authors who can create a strong visual in the reader's mind with just a few words are remarkable. Also, stories that touch on social issues in a subtle yet impactful manner are great. They can make the reader aware of a problem without being preachy.
One must - read story could be 'Flashpoint'. It completely alters the DC Universe's timeline in a really interesting way, with the Flash at the center of the chaos. It shows a darker side of the Flash's world and has some great art to go along with it.
I'd recommend 'A Glimpse of Hope'. It's a great example because it manages to tell a complete and engaging story within a limited word count. The plot twist at the end is truly unexpected.
There are several sources for must - read flash stories. Bookstores, especially those that have a good selection of short story anthologies, are great places to look. Additionally, some literary magazines, both in print and online, often publish flash stories. For example, 'The New Yorker' sometimes publishes excellent short pieces that could be considered must - reads.
Some of the must - read stories could be the ones that explore future technologies in a very unique way. For example, stories about artificial intelligence gone rogue or space exploration to distant galaxies.