Well, it depends on various factors. Comics that are in mint condition, have significant historical or cultural importance, or are from limited print runs tend to be worth a lot. For instance, early issues of Marvel's X-Men or DC's Batman can fetch high prices if they meet these criteria.
It's hard to pinpoint just one. But certain comic books like Amazing Fantasy #15, which introduced Spider-Man, or Detective Comics #27 with the first appearance of Batman, are often considered highly valuable due to their scarcity and influence on the comic book industry.