Action Comics No. 1 — originally released in 1938 — marked the first appearance of Superman. And a copy authenticated and graded 6.0 by CGC was just sold for a cool $3.18mm USD.
The particular issue is called "The Rocket Copy" by those in the collecting and alternative investment communities, as a subtle rocket stamp was placed on the cover by the 13-year-old who purchased it from the newsstand in 1938. As for the sale itself, it was brokered by Goldin on behalf of a private client.