Warning: Will include spoilers for the comic series that may or may not appear on future television episodes.
1. Showcase #8, "Mystery of the Human Thunderbolt / The Man Who Broke the Time Barrier" (October 1956)
2. The Flash #110, "Challenge of The Weather Wizard!" (January 1960)
3. The Flash #127, "Reign of the Super-Gorilla!" (March 1962)
4. The Flash #123 "Flash of Two Worlds!" (September 1961)
5. The Flash #133, "Plight of the Puppet-Flash!" (December 1962)
6. The Flash #139, "Menace of the Reverse-Flash!" (September 1963)
7. The Flash #140, "The Heat Is On... For Captain Cold!" (November 1963)
8. The Flash #163, "The Flash Stakes His Life On -- YOU!" (August 1966)
9. The Flash #174, "Stupendous Triumph of the Six Super-Villains!" (November 1967)
10. The Flash #184, "The Executioner of Central City" (December 1968)
11. The Flash #275, "The Last Dance!" (July 1979)
12. The Flash #276, "Freakout!" (August 1979)
13. Crisis on Infinite Earths #8 "A Flash of the Lightning" (November 1985)
14. The Brave and the Bold #28, "Starro the Conqueror!" (March 1960)
15. Justice League of America #1, "The World of No Return!" (August 1960)
16. Justice League of America #9, "The Origin of The Justice League!"
17. Superman #199, "Superman's Race with The Flash!" (August 1967)
They were literally. Obsessed. With. Racing.
Just whip them out and get a ruler already, boys.
Retail values found at Comics Price Guide and eBay.