I should probably think back and try to do this for Superman and Batman, but I just had this idea today after reading DC Comics' post on Google+.
So this was my first Flash comic book, The Flash #231.
I got this book from a friend of my dad who ran a drugstore and gave me a bunch of books that hadn't sold.
It was a crash course introduction to the Rogues Gallery for me, and also a great example of just how fast the Flash is. The second story in the book with Green Lantern also introduced me to Aaron Burr years before the famous Got Milk commercial.