According to the Guinness Book of World Records, it was Marvel Comics #1, sold for $350,000. According to the Overstreet Price Guide, it was Action Comics #1 at $350,000, followed by Detective Comics #27 (the first appearance of Batman in print), at $300,000.

The world’s most valuable comic book is likely Action Comics #1 (June 1938), the first appearance of Superman. This issue essentially ushered in the Golden Age of Comic Books and began the superhero genre. Less than a hundred copies are known to exist. At $350,000, it topped the Overstreet list. In 2003, the head of Diamond Comic Distributors offered one million dollars for a near-mint copy, although a copy in such good condition has never been discovered. http://www.answers.com/topic/first-appearance-1 1. Action Comics #1 $350,000 2. Detective Comics #27 $300,000 3. Marvel Comics #1 $250,000 4. Superman #1 $210,000 5. All-American Comics #16 $115,000 6(Tie). Captain America Comics #1 $100,000 6(Tie). Captain America Comics #1 $100,000 8. Flash Comics #1 $85,000 9. Whiz Comics #2 (#1) $77,000 10. More Fun Comics #52 $72,000 As reported in THE OVERSTREET COMIC BOOK PRICE GUIDE #32,

The value of a book can be greater than the price it sells for, as determined by those who trade them as trinkets for the wealthy, particularly when one considers the extremely inflated prices paid for 20th-century comic books and other samples of sequential art. Early forms of sequential art are more valuable than mid- to late-20th century comic books, because they show the evolution of a form of art and communication that has been in use for centuries. Many of the features found in modern comics were created hundreds of years ago. Early works date back to the Middle Ages and are extremely rare. These early examples show how sequential art was used to disseminate religious stories, stories of saints and other religious personages, popular stories, and humorous or satirical tales. The stories would be recounted by those who could read to those who could not. The majority of the people in the target audience were unable to read, but could follow along with the pictures. This proved an effective means of disseminating information at a time when most were illiterate. Most of these works are long gone, because they wore out from use or were disposed of without thought for future historians - in much the same way as 20th-century comic books. These works have an importance beyond their historical value as examples of early forms of sequential art. They are a snapshot of the social and political mores of the time. These historical works are held in museums and by private collectors. The monetary value cannot really be estimated, because they are irreplaceable. They are not a commodity traded for all the market can bear - and then some. ------------------------------------------------------------ Re: "I think the questioner had Spider Man and the likes in mind - considering the category." Perhaps, but this is the only category on Answerbag that discusses sequential art in all of its forms. I wrote this answer to serve two purposes. First, the value of an object is not determined by the number of dollar-pounds dropped on the table in exchange for permission to take it home. It is simply one facet, but in a market where such items are treated as profit-making trinkets of jewellery - far from the original intent of comic books - I don't feel that a wildly inflated price is any indication of intrinsic value. Secondly, the importance - another form of value - of an item may not always be apparent and is seldom reflected in its cost. If we look at the development of the modern sequential art form, from the appearance of colour comic supplements in the 1890s, through pulp comic books, and finally to the graphic novel, all haved played a role in society unrelated to their monetary value. When we look at one of the earliest regulars in the colour supplements in the US, "Hogan's Alley", we meet the Yellow Kid. Why is the Kid remembered? The expressions "yellow press" and "yellow journalism" stem directly from "Hogan's Alley" and the Yellow Kid, when the strip was used help to fan the flames of American involvement in the Spanish-American War. This was a war that largely resulted from a subscription war between Hearst and Pulitzer. It might never have occured were it not for the patriotic fervour whipped up by these papers. The "yellow press" is the cheapest form of journalism, in which fright headlines, sensationalistic and inaccurate reporting, and gaudy pictures are used to hammer the readership into line with the paper's editorial position. Where have we seen this sort of thing recently? And beside all of this, the money that exchanged hands to purchase a copy of Joe Shuster's "Superman" seems strangely irrelevant.

It's Action Comics #1. http://www.nostomania.com/servlets/com.nostomania.CatPage?name=Top100ComicsMain $994,000