2) Brotherhood Of Mutants
Few supervillain team are more synonymous with their superhero rivals than the Brotherhood of (Evil) Mutants. First appearing all the way back in 1964’s Uncanny X-Men #4, the Brotherhood are of course the darker, more militant counterpart to the X-Men. Just as they’re mentored by Charles Xavier and follow in his beliefs of peace and unity, the Brotherhood follow Erik Lensherr AKA Magneto and share his view that mutantkind is superior to humanity.
Over the years, the Brotherhood has grown from the likes of Toad, Mastermind and even future Avengers Scarlet Witch and Quicksilver to include just about every major mutant threat to the X-Men. Notably later members include Mystique, Sabretooth, Blob, Pyro, Black Tom Cassidy and Sauron (not The Lord of the Rings one).
In that way, the Brotherhood are often presented as a more understandable, even sympathetic, group than most comic book supervillain teams. On many occasions, Magneto’s men have allied themselves with the good guys. Just like the X-Men, the Brotherhood only want to save their own people. They just have a more violent way of doing it.