The Marvel Universe has many outstanding super heroes and teams, and a plethora of villains for them to face. But some of these evildoers transcend mere "baddie-of-the-month" status and pose a fearsome challenge for our beloved good guys.
The characterizations and motivations of each may differ but the end goal for each of them is the same: exterminate their arch-nemesis!
I know I have left some out but I am limited to nine choices. Feel free to name your own favorite hero/villain adversaries.
Avengers - Kang
The Avengers have faced off against numerous enemies but time and time again it is Kang or one of his counterparts (Rama-Tut, Immortus, Scarlet Centurion, Council of Cross Time Kangs) that pop up with a sinister scheme. The Avengers usually win, but at a cost.
Other strong candidates are: Ultron, Thanos, Masters of Evil
Spider-Man - Green Goblin
There have been a few different Green Goblins and also a handful of Hobgoblins but Norman Osborne is the ultimate thorn in Spider-Man's side. The wall-crawler has dozens of foes but none have plagued him for as long and as effectively as has the one true Green Goblin.
Some more: Kingpin, Doctor Octopus
Fantastic Four - Dr. Doom
No doubt here, Victor Von Doom is the premiere opponent of the Fantastic Four. Sure, he's fought every hero there is but his obsession with proving he is better than Reed Richards drives him towards destroying him and his family.
Honorable mention: Galactus, Mad Thinker, Mole Man
X-Men - Magneto
Linked from day one, Magneto is at odds with Professor X over the course of action mutants should follow. This leads to constant conflict with all of the respective X-teams.
Can't forget: Mr. Sinister, Juggernaut, Hellfire Club
Hulk - Leader
This big headed guy is constantly hounding Hulk and won't leave Hulk alone. Why won't Bighead just go away?
Not going to leave out: Abomination, M.O.D.O.K., the US Army
Dr. Strange - Dormammu
Ruler of the Dark Dimension and master of the mystic arts. This mage with the fiery noggin continually tries to usurp Dr. Strange's power and take over our plane of existence.
Can also say: Baron Mordo, Nightmare
Captain America - Red Skull
A thug who stands against everything that Captain America believes in, the Red Skull's machinations have caused Steve Rogers' alter ego untold amounts of pain and frustration throughout the years.
Plus: Secret Empire, Baron Zemo (either one, take your pick)
Iron Man - Mandarin
The Mandarin has crossed paths with Tony Stark on many occasions and always proves to be a handful. His M.O. of sending waves of cannon fodder to wear Iron Man down has come close to working a number of times.
S.H.I.E.L.D. - Hydra
Every super-spy organization has to have a deadly rival group and Hydra is the KAOS to S.H.I.E.L.D's CONTROL. The epic confrontations between these two groups have leveled islands, flattened cities and sent many a heli-carrier down in flames.
I would be remiss if I didn't also mention: AIM, Scorpio
I didn't have room for other heroes and their nemeses (Daredevil - Kingpin; Wolverine - Sabretooth; Thor - Loki; Howard the Duck - Dr. Bong; etc.) so please feel free to add your own. And if you disagree or want to add something to me selections then have at it!
Excelsior!
I am proud to Announce that I now have 500+ Hero Ratings!!! Thanks to all of my Amazing Customers! Big things are in store for Hall of Heroes this year. It is my personal goal to add a long box a week to my Inventory starting in April, 2014.
Please frequent the VS Forum located here. Have you ever wondered if so-and-so could beat blankety-blank? Well, wonder no longer. Everything can be decided here. 
Watch Turn on AMC starting Sunday, April 6th!