Magneto's costume, like the Master of Magnetism himself, has many layers. One the surface, it is a very pleasing color combination of red and royal purple, tight fitting and flattering to his superb figure:):)
And since, according to Claremont, it's made up of metallic fibers, it certainly suits and even flaunts his vast power. Magneto intends to be intimidating and the costume certainly lends itself to that purpose with the sinister, concealing helmet, et al. But the helmet is also VERY reminescent of the helm of a Spartan hoplite ... *fierce* warriors who were rarely bested in combat! The flowing cape both lends an air of great dignity, gravity and even granduer to the ensemble.
This is a rather nifty link with his name, actually! "Magnus". In Latin Magnus means, "Great One" and the cape certainly reinforces that image! Again, like the cape of a Spartan warrior it is red. Spartan hoplites wore red so that their enemies would never be favored with the sight of Spartan blood. Just so with Magneto:):)
The royal purple of the costume (a color worn only by Kings in the ancient world by virtue of the scarcity of the dye needed to produce it) once again, alludes strongly to the name. Red, of course, is also the color of blood ... and Magneto's past, as a survivor of the Holocaust, is drenched in blood. The blood red is a very visible and startling representation of his mission: NEVER AGAIN! While he wears that costume he will never forget his mission. He won't let himself. And, at one and the same time, it is a stark reminder of the blood he himself has spilled in his quest to protect mutant kind! He's not likely to forget that either. It is a perfect, encompassing symbol of the yin/yang that is Magneto.
And that's why I think Magneto has the best costume in the MU:):)