Thanos VS Magneto:
Before catching the briefest glimpse of Thanos in the end credits of the Avengers film, most mainstream audiences probably hadn't even heard of the mad titan. But longtime Marvel readers know him as the cosmic badass who long possessed the reality-altering Infinity Gauntlet, and who slew half the universe, including many Marvel heroes, before his misdeeds were undone. He just closed out an appearnace in Infinity, where Thanos once again threatened the Earth in pursuit of his nefarious goals, Thanos will most likely rise to prominence as one of the villains in Marvel's upcoming Guardians of the Galaxy, part of its second wave of films.
Magneto has straddled the line between hero and villain over and over again since his introduction in the '60's, and his current status quo places him somewhere in the grey area between the two extremes, but in his villainous days Magneto has often been one of Marvel's most ruthless villains. It's Magneto's dichotomy that makes him so compelling, and so effective as a villain. It's hard not to sympathize with Magneto knowing his suffering at the hands of the Nazis, and understanding that his extreme views have often been pursued while attempting to build a future for mutant kind.