The Marvel Universe (that's Universe 616) is chalk full of heroes and villains. But the most compelling battles are between those characters who have clashed time and time again, inflicting massive damage - both physical and psychological - to each other. One of the major challenges in presenting characters that have faced off for decades in real time is keeping things fresh. It can be really tricky to not allow things to become repetitive and tiresome. It's for this reason that you'll notice the absence of some of the more famous rivalries - you won't see Captain America (by which I obviously mean Steve Rogers) vs The Red Skull here for the precise fact that I see their continuous battle as stale and boring. So here's my best stab at those enemies whose eternal strife has been the most significant and entertaining over the years.
5. Mister Fantastic vs Doctor Doom
I like this one because it extends far beyond any physical struggle. Sure, they have tangled that way, matching super stretchiness against Doom's various technology-infused abilities but mostly with these two, it's mind versus mind. Doom is easily the Marvel U's biggest ego - his megalomania extends far beyond that of any villain I'm aware of - and he actually views the great mind of Reed Richards as woefully inferior to his own. He maintains this belief despite being defeated by the FF - almost always because of Richards' brain - over and over. No matter what happens, Doom can find an excuse for it - nothing is ever his fault as he's always above error.
I'll admit I'm not too versed in Fantastic Four lore so I can't really bring up any specific examples of Richards vs Von Doom but I still count their rivalry as one of the absolute best.
4. Daredevil vs Bullseye
One might argue that the Kingpin is a greater nemesis for DD than Bullseye since a lot of Bullseye's initial blows against him came courtesy of the giant crime lord. But in this case, it's the physical battle that I really like. And Wilson Fisk is really no physical match for The Man Without Fear. But Bullseye is perfect. His abilities are extremely similar to those of Murdoch and Daredevil's enhanced senses are the ideal foil for the assassin's out-of-this-world aim. Being able to dodge Bullseye's attacks with regularity was more than enough to drive him nuts and the guy was already plenty nuts to begin with. And, as I mentioned earlier, while their rivalry began because of the Kingpin, it became deadly personal very early on with Bullseye killing Daredevil's sort-of love interest, Elektra in a battle to determine who was the number one assassin. And THEN, all on his own, Bullseye goes on to murder a much more stable love interest of Murdoch's, Karen Page.
The two are so remarkably similar and yet such polar opposites they make perfect enemies. An especially great illustration of this is an arc in which the two trade costumes and identities as they slug it out, each attacking the other emotionally as much as physically.
3. Professor Xavier vs Magneto
Former friends often make for the most compelling of enemies. After first meeting in Israel, these two together dreamed of a world where mutants and ordinary humans could live side by side in peaceful harmony. But the harsh reality of most humans shunning and downright despising mutants soon was too much for Erik Lehnsherr to bear and he quickly adopted an "us against them" mentality, ultimately deciding that the only way for mutants to get along in the world was to dominate the humans. He also adopted the title, Magneto, Master of Magnetism. Xavier believed it was best to continue his efforts towards a peaceful coexistence and Magneto became one of the biggest obstacles standing in his way. Over the years, the two have faced off countless times and some encounters have left very deep scars, perhaps the most memorable being Magneto magnetically ripping the adamantium from X-Man Wolverine's skeleton, very nearly killing him while putting him through unimaginable physical agony. This prompted Charles to do something he thought he would never resort to - use his telepathic gift to erase the mind of his longtime foe. As it goes in the world of comics, both Wolverine and Magneto recovered from these injuries but the emotional scars will always remain. The Age of Apocalypse timeline showed that in a world where Xavier had perished, Magneto could be a hero carrying on the ideals of his friend but the normal continuity drives home that while the two coexist, they will forever be enemies.
2. Wolverine vs Sabretooth
While this pair were never exactly friends, they did at least once fight on the same side. In the sixties as agents of the Canadian government's secret team, Team X, they were deployed together on lots of missions and mostly got along. But after a disastrous encounter with Russian super-soldier Omega Red, where Sabretooth followed team protocol leaving behind teammates in an escape, Wolverine was furious. The two had actually been enemies decades before with Sabretooth raping and seemingly murdering Logan's lover, Silver Fox. But when they joined Team X, both of their memories had been altered and they didn't remember each other. But eventually they both came to remember plenty and have had tons of battles, all intense and vicious. Even besides their awesome history together, they make for such great enemies because of their extremely similar powers and natures. Sabretooth is stronger and faster but Wolverine has always been able to make up for this by being craftier- when they fight, it's usually to a standstill but only after some ferociously brutal action.
1. Spider-Man vs The Green Goblin (Norman Osborn)
No two enemies within Marvel have had a more personal and devastating struggle. Since almost the very beginning of the web-slinger's career as a costumed hero, The Green Goblin has been around to torment him. Fighting him as the heavily armed, halloween-themed Green Goblin and also employing other villains to attack his foe, Osborn's assaults against Spider-Man were the most obsessive and relentless of all until Venom showed up. Father of Parker's close friend, Harry, Norman Osborn was the very first villain to deduce Spider-Man's civilian identity and he exploited this advantage to the fullest. Parker would also eventually learn the identity of his nemesis and let me tell you, it made for some pretty awkward moments. This would culminate in the Goblin's ultimate strike against Spider-Man, the murder of Parker's girlfriend, Gwen Stacy. With Spider-Man looking on, Osborn threw Stacy off Brooklyn Bridge to her death. This prompted Parker to beat the Goblin nearly to death before coming to his senses. Then Osborn wound up impaling himself with his glider in an attempt to skewer his enemy. And so he was believed to be dead.
But his legacy lived on. Harry would lose his mind and take up the mantle of the Green Goblin as well and a chance discovery would lead to several Hobgoblins and even a demonic Demo-Goblin from hell.
Now I'm usually strongly against the resurrection of a dead character, especially a major one like Green Goblin, but I must admit his return to life in comics was actually a stroke of genius. And the reveal that, while believed dead and working from the shadows, Osborn was actually responsible for the entire clone saga gave that annoying storyline some punch and meaning. In a less cool reveal, it's shown he'd also seduced and impregnated Stacy before killing her. And since then, Osborn has gone on to elude justice, even once his identity became known. This is such a mockery of all Spider-Man has worked for and suffered through that it ramps up the hatred and intensity all the more. Now with the Skrull invasion averted and Osborn seen as a hero, you can bet the rivalry between the Goblin (oh, he's still in there) and the Spider will only grow.