Time Travel is something that gives superhero comics the most obvious tie to their science fiction roots. In Avengers: Endgame, it finally found its place in comics' mainstream crossover into feature films.
In Marvel Comics, time travel was initially defined as not being linear. If someone traveled to the past or future, then they traveled to an alternate timeline because they couldn't create a paradox. Recently, that has changed, giving stories a bit more drama with increased consequences.
10 Fantastic Four #5
This is probably the earliest instance of time travel in the Marvel Universe. Doctor Doom's time machine remains the go-to standard within the storytelling of the Marvel Universe. Doctor Doom first used it to send the Fantastic Four back for Blackbeard's treasure chest, which contained a gem he needed. Of course, they fooled him and thwarted his plans.
What's amazing is how fun this early comic is. It does play loose with history. The Thing does lose himself in the role of Blackbeard the Pirate. Given how new he is at being a monster among men, his reasoning makes sense. It also helps shape the mental state of Ben Grimm at this point in the Fantastic Four's development. It's almost enough to make you forget that Blackbeard was a real person in history.
9 Age of Ultron
As a whole, the event Age of Ultron wasn't completely successful as a result of a solution hinging on time travel. It was interesting in the way it showed the problems of using time travel as a solution. It tried to keep to the established rules of time travel in the Marvel Universe but does play with using time travel to change the present.
Wolverine and Invisible Woman go back in time initially to talk Hank Pym into not building Ultron. Wolverine tries killing him, and he finds that he opens up a present that changed for the loss of Hank Pym. Going back again, they instead provide a way to stop Ultron closest to their point of departure. It's the plot of Avengers: Endgame but in a handful of issues instead of a three-hour film.
8 X-Men #41
Charles Xavier's son Legion went into the past to kill Magneto. It's the whole "kill Hitler before he starts World War II" scenario. Like with most time travel stories of this nature, something goes wrong. In this case, Charles Xavier throws himself in the way and dies instead of Magneto. The result is the Age of Apocalypse.
The world without Charles Xavier is much bleaker than the one Legion left behind. Magneto forms a team of X-Men very different from what we knew. Readers were left wondering how Marvel was going to fix things. Bishop goes back to prevent Xavier from dying, restoring the timeline.
7 The Adventures of Cyclops and Phoenix
If the art of Gene Ha isn't enough of a reason to buy Adventures of Cyclops and Phoenix, there is the story of Cyclops and Jean Grey getting a chance to raise Cable. They train him in the use of his powers, especially in keeping the techno-organic virus at bay.
All the time, Cable never learns that he was raised by his father and genetic mother. Cyclops and Phoenix do this for twelve years, finally sending their minds back to the present after assisting Cable in defeating Apocalypse. It provides the most appropriate honeymoon for this newlywed couple.
6 Doomquest
This story ran in Iron Man #149 and #150. Iron Man seeks to reverse an arms sale to Doctor Doom. Along the way, the pair find themselves thrown back to Camelot by Doom's Time Platform. Doom sides with Morgan Le Fey, and Iron Man, of course, aids King Arthur. Eventually. the two combine their armor technology to create an ersatz time machine and return home.
This story establishes Tony Stark and Doctor Doom as two of the smartest men in the Marvel Universe. It also uses the fact that Doom has a time machine at his disposal to attain his goals. Unfortunately, it ignores some of the established continuity that Doom is not the ruler of Latveria at this point. As long as a reader takes it for what it is, it remains an excellent Marvel time travel story.
5 Hulk: Future Imperfect
The Hulk gets brought to the dystopian far future to stop its despotic ruler, the Maestro. After being captured, Hulk learns that Maestro is an older, more cruel version of himself. He has to figure out how to topple this cruel despot who is every bit his equal, if not his superior.
We also see lots of Easter Eggs in the various mementos Rick Jones keeps. Hulk makes use of the rules of time travel because this is"Professor Hulk." Of course, being drawn by George Pérez, we'd expect to see lots of little Easter Eggs, especially if we can't get massive group shots.
4 Further Adventures of Cyclops and Phoenix
After the Adventures of Cyclops and Phoenix did so well, a sequel seemed inevitable. A sequel is rarely better than the original, and the Further Adventures of Cyclops and Phoenix. It puts the couple in the past instead of the future, where they must again stop Apocalypse, awakening a century earlier than they expected.
The story serves primarily as an origin story for Mister Sinister. His fascination for the Summers and Grey lineage gets its roots here, and he finds himself at odds with Apocalypse, which firmly makes him a sympathetic character for perhaps the first time. It doesn't hurt to have Victorian England rendered with the moody style of John Paul Leon.
3 Avengers Forever
If there's going to be a time-traveling adventure pulling Avengers from all eras, including the future, then it better be a twelve-issue series. Avengers Forever brings the Avengers in partnership with Kang the Conqueror. It suitably has breathtaking spreads by Carlos Pacheco.
The brilliant part of it is the assemblage of Avengers. While Goliath and the Wasp are from the present day, Hawkeye is from just after the Kree-Skull War. Captain America is from just before he takes the identity of Nomad. Yellowjacket is from just before his wedding and unaware he is Hank Pym. Songbird and Captain Marvel are from the future. They all work together to keep Immortus and the Time-Keepers from destroying humanity rather than having it conquer the stars.
2 Fantastic Four #291-292
It's not a high-profile story, but the two-issue story that spanned Fantastic Four #291-292 was a great sense of time travel by John Byrne that worked the time travel story in a new way. With Reed Richards believed dead, the remaining members of the Fantastic Four find themselves trapped in 1936 with Nick Fury.
It works by making a shift at the end to reveal things are not as they seem, but foreshadowing it in the first issue. It works that Fury would set off to kill Hitler and risk changing everything to save millions of lives. Along the way, we also get the commentary of how UPS trucks look out of date.
1 Days Of Future Past
Of course, Days of Future Past would top the list. Chris Claremont and John Byrne gave a new way of traveling in time. We also see a dark glimpse of the future for mutants and humans alike. It's paired with the pivotal moment that an adult Kate Pryde comes back to change.
Of course, she changes it, but readers are left wondering if the future, where they just watched all of the X-Men die, did change at all. Of course, with the established way that Marvel time travel works. It didn't change, and we see that in the following years, with Rachel Summers making her way into the present, and Excalibur traveling to her world.