The X-Men, a group of incredibly powerful mutants, fight to save a world that hates and fears them. Logically, they would have a lot of enemies, both mutants and humans alike, who disagree with their mission. Humans don't understand their power and fear what they might do with it, and mutants believe they are the superior beings and see the X-Men as traitors who just get in the way.
When going up against an entire group of mutants, be it just a few or a whole gaggle (The X-Team gets huge at times) it's only logical to form a team yourself. Why go up against an army solo?These are those mutants and humans who have formed teams to take down the children of the atom. These are the greatest teams of villains to go up against the X-Men!
Brotherhood of Mutants
The very first team/combatants of any kind to go up against the X-Men was the brotherhood of mutants. This team, originally led by Magneto, believes that mutants are the superior race and that humans should work for them or die.
Members include:Toad, Quicksilver, Mastermind, Scarlet Witch, Blob, Unus, Pyro, Destiny, Avalanche, Rogue, Sauron, Mimic, Sabretooth, Juggernaut, and many others. The team has also been led by Mystique.
Horseman of the Apocalypse
Apocalypse enslaves four mutants to do his bidding and enact his survival of the fittest credo on humanity. The horseman become either Pestilence, War, Famine, or Death. Many mutants have held these titles. Wolverine was even Death.
Members include:Archangel, Caliban, The Hulk, Wolverine, Deathbird, Ahab, Gazer, Polaris, Sunfire, Gambit, and more.
The Hellfire Club (The Inner Circle)
An extremely powerful, long standing club with many powerful and political members. Many members not in the Inner Circle (like Tony Stark or even Norman Osborn) may not know the full depths of depravity the group gets into.
The Inner Circle includes:Sebastien Shaw, Emma Frost, Sir Gordon Phillips, Sir Patrick Clemens, Lady Diana Knight, Sage, Candy Southern, Selene, Harry Leland, Madelyn Pryor, Donald Pierce, Trevor Fitzroy, Mastermind, Sir Henry Manners, and others.
First assembled by Mr. Sinister to purge the world of the "inferior" Morlocks.
Members Include:Arclight, Blockbuster, Gambit, Hans, Harpoon, Lady Mastermind, Malice, Mystique, Omega Sentinel, Prism, Riptide, Sabretooth, Scalphunter, Scrambler, Sunfire, and Vertigo.