Early Issues:
* Amazing Fantasy #15 (1962): The first appearance of Spider-Man.
* Incredible Hulk #1 (1962): The first solo Hulk issue.
* Fantastic Four #1 (1961): The birth of the Fantastic Four and the start of the Marvel Universe.
* Tales to Astonish #27 (1961): The first appearance of Ant-Man.
* X-Men #1 (1963): The inaugural issue of the X-Men.
Key Issues and First Appearances:
* Avengers #1 (1963): The first appearance of the Avengers.
* Daredevil #1 (1964): The first appearance of Daredevil.
* Iron Man #1 (1968): The first appearance of Iron Man.
* Thor #126 (1965): The first appearance of Beta Ray Bill.
* Captain America #217 (1978): The first appearance of the Winter Soldier.
* Secret Wars #8 (1984): The first appearance of Venom.
Other Notable Comics:
* The Amazing Spider-Man #129 (1974): The "Death of Gwen Stacy" issue.
* X-Men #141 (1981): The "Dark Phoenix Saga" begins.
* Daredevil #181 (1982): The first appearance of Bullseye.
* Amazing Spider-Man #300 (1988): A milestone issue with a major story arc.
Factors Affecting Value:
* Condition: A comic in pristine condition (CGC 9.8) is significantly more valuable than one with wear and tear.
* Rarity: Limited editions, special prints, or comics with low print runs are usually more valuable.
* Demand: Popular characters, significant storylines, and issues with strong cultural impact are more sought after.
Remember: It's crucial to consult with reputable comic book dealers and experts for accurate valuations. Prices can vary greatly, and a comic's value is a complex assessment.
A good resource to find the estimated values of Marvel comics is the CGC Comic Price Guide**.