I think the key books with the highest potential for value increases would be titles related to Thanos, as well as Thor #344 and Guardians of the Galaxy titles.
I've always wanted a Spider-Man #129 myself (1st Punisher) but they're so ungodly expensive already.
Key Titles to keep an eye out for would be
Marvel Preview #7 (1976) if you want Rocket Raccoon’s first.
Iron Man #55 – The First appearance of Thanos. If there was ever one comic to buy for your collection right now, this would be It. (VERY expensive now)
Marvel Super-Heroes #12 – Featuring Captain Marvel! The first Captain Marvel. Captain Marvel could be the answer to DC’s Superman. The potential upticks here for Captain Marvel is simply astounding. Buy this.
Fantastic Four #48 – 1st Watcher, Silver Surfer AND Galactus also make their debut here.
The Infinity Gems – It’s very safe to assume that Thanos will be seeking the gems in the second Avengers film. Here are some of the comics that feature the first look at these gems that the madman craves.
Power of Warlock #1
Marvel Team-Up #55
Avengers Annual #7.
Incredible Hulk #234 – First appearance of Quasar.
Strange Tales #138 – First appearance of Eternity.
The Eternal #2 – The Celestial first appear here.
Captain Marvel #27 – Death first appears here.
Tales of Suspense #79 - 1st look at the Cosmic Cube
Secret Wars #1 – The Beyonder makes his debut here.
Quasar #24 – First appearance of Infinity.
Giant Sized Defenders #3 – First look at Korvac.
Fantastic Four #20 – First Molecule Man.
X-Men #15 – First appearance of Onslaught.
Fantastic Four #18 – First Super-Skrull.
Thor #344 - 1st Dark Elf (Movie villain)
There are more, but this should give you start on some cool books to look out for
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