The X-Men's earliest years under team creators Stan Lee and Jack Kirby aren't a bright spot in the book's history. The mutant allegory is feels half-baked at times, reeking of creators concerned about sitting too far on one side of the fence, while the team dynamic between the original five X-Men is a pale imitation of Lee & Kirby's first superhero team, The Fantastic Four.
The book's earliest villains aren't much of a bright spot either, lacking depth with powers and costumes ranging from gaudy to outright dull. Taking a look at the growing pains of the earliest X-Men issues through the lens of the first ten villains the team faced can help us appreciate how far the team has come.
10 They Fought Magneto in X-Men #1
One aspect of the earliest X-Men comics which has endured the test of time is Magneto as the team's arch-enemy. Appearing right out the gate in X-Men #1, the Master Of Magnetism's plan to nuke the Earth is foiled by the titular heroes. His appearances from there don't let up; he's not just the X-Men's most powerful foe, but their most recurring.
However, Lee & Kirby's Magneto is only a faint echo of Chris Claremont's later version; he lacks any personal history with Professor X or his tragic backstory as a Holocaust survivor. His plans are motivated not by a refusal to allow mutants to be butchered like his family and people were by the Nazis, but by a narcissistic belief in Homo Superior's innate dominance.
9 They Fought The Vanisher in X-Men #2
The second issue features the X-Men square off against a teleporting mutant thief known as the Vanisher. After contenting himself with bank robbery, the Vanisher uses his powers to infiltrate the Pentagon and steal military secrets he can auction off. He's foiled not by the student X-Men, but the teacher; Professor X telepathically induces amnesia and the Vanisher is then arrested.
Though he's made recurring appearances since, the Vanisher has never ranked high enough in the X-Men's rogues that one would guess he was their second adversary.
8 They Fought The Blob in X-Men #3
X-Men #3 features a team recruitment gone awry. After Professor X senses carnival performer Frederick "The Blob" Dukes is a mutant, the team offers to add him to their ranks. The Blob rejects their offer and turns on the team when Xavier attempts a brainwash of him.
The Blob has been a perennial X-Men villain since his debut, though he's invariably a C-Lister and usually a henchmen of a more powerful villain. With his power-set and appearance, there are limits to how threatening he can be.
7 They Fought The Brotherhood Of Evil Mutants in X-Men #4
Magneto returns in Issue #4, now leading a team of mutants to counter the X-Men. The ranks of his Brotherhood consist of the deformed Toad (a very literal toady for Magneto), the illusionist Mastermind, and the twins: super-fast Quicksilver & probability-altering Scarlet Witch. In their debut issue, the team attempts to subjugate island micro-nation San Marco into a mutant homeland, but they don't stop once that plan fails.
In the next issue, they kidnap Angel to give themselves a numbers advantage, then in issues #6 & 7 they add the Sub-Mariner and the Blob, respectively, to their ranks. After a guest appearance battling Thor in Journey Into Mystery #109, the original line-up is dissolved in Issue #11 once Magneto is abducted (more on that to come).
6 They Fought Namor The Sub-Mariner in X-Men #6
The Silver Age features Namor The Sub-Mariner at his most overtly villainous - after returning in the pages of Fantastic Four, he was frequently used as a villain across varying Marvel titles.
His bout with the X-Men came in Issue #6 of their series, when he joins the Brotherhood of Evil Mutants before realizing the depths of Magneto's depravity. Notably, this issue retconned Namor into having been a Mutant all along; though only a peripheral part of the X-Men's world, this aspect of his character has been abided by since.
5 They Fought Unus The Untouchable in X-Men #8
Issue #8 followed the formula of the last two; the Brotherhood of Evil Mutants (in this issue represented only by Magneto and Mastermind) recruit a mutant and pit them against the X-Men. Unlike the last two, the mutant in question was a character readers had never met before: Unus The Untouchable, capable of projecting invisible force-fields around himself.
Unus is told he must defeat one of the X-Men before he can join the Brotherhood, but after his powers spiral out of control, he barters with Beast to repair them in exchange for not opposing the X-Men.
4 They Fought Lucifer in X-Men #9
After Issue #8, Lee & Kirby took a break from the Brotherhood, and indeed from evil mutants altogether. This issue saw the debut of Lucifer, an alien Quist sent to Earth as the prelude of an invasion. In his debut appearance, he tricks the Avengers into battling the X-Men, while when he returns in Issue #20, he's revealed to have a history with Professor X; it's him who paralyzed Xavier.
Despite this important role in Xavier's backstory, Lucifer never quite took off as a villain and has been unseen entirely in recent decades. However, the idea of a villain having a history with Professor X has since been more successfully transplanted onto Magneto.
3 They Fought Maa-Gor in X-Men #10
Issue #10 of the X-Men saw the debut of Ka-Zar, Marvel's riff on Tarzan of the Apes. It also introduced the Savage Land, a primordial land filled with prehistoric creatures stuffed inside Antarctica. The villain of the issue is Maa-Gor, another Savage Land inhabitant and a member of the "man-ape" species.
While Ka-Zar would develop into a character mostly divorced from the X-Men, the Savage Land has been a recurring setting for the team's adventures since.
2 They Fought The Stranger in X-Men #11
The Brotherhood returns in Issue #11. The story initially seemed to be going through the motions established in Issues #6-8; the Evil Mutants recruit a new member to their ranks. However, the recruit in question, known as "The Stranger," turns out not to be a mutant, but an intergalactic entity who leaves Earth with Magneto and Toad as his captives. With their leader gone, the Brotherhood disbands.
The Stranger promises that Magneto shall never return, but fails to keep his word - Magneto escapes in X-Men #17 then again in Avengers #47, at which point the Stranger stops trying to keep him captive.
1 They Fought Juggernaut in X-Men #12
The X-Men's relief at having their arch-enemy gone didn't last. Issue #11's ending featured Professor X warning Cyclops that a threat was approaching - Issue #12 revealed this to be the Juggernaut, the Professor's step-brother Cain Marko.
Infused with super-strength and stamina by the mystical gem of Cyttorak, the X-Men battle the Juggernaut over the course of two issues; in the end, they need the Human Torch's help to defeat him. Aside from Magneto, the Juggernaut is easily the most enduring of the X-Men villains introduced up to this point.