A true timeless classic, and one that’s been adapted and retold multiple times in both other comics and cartoons. It’s the one where Flash dresses as Spidey and gets nabbed by the bad guys.
It’s also the first crossover between two different Marvel comics that clearly fits in continuity. It picks up where Fantastic Four #17 left off, with Doom jumping out of a plane to escape the FF…
Doom then has a run-in with Spidey.
This is a huge step forward for the “shared universe” concept. The only thing missing is the editor’s box telling the reader that this happened in FF#17.