This volume reprints four of the top 100 Marvel comic books ever, as voted on by fans. They are all great issues; the only problem is that some of them are part of multi-issue arcs, so you don't get the full story in one volume. Still, these are some of the best and most important comic book issues ever, and are definitely worth reading for any Marvel fan.
Issues in this volume (and their importance):
21. The Avengers (vol. 1) #1 First appearance of the the Avengers
20. Uncanny X-Men #350 Gambits secret revealed, return of Magneto
19. The Amazing Spider-Man (vol. 1) #122 Death of Green Goblin (Norman Osborn)
18. Captain America (vol. 1) #109 Origin of Captain America retold