As someone who actually liked the original X-men movie's costumes apart from the fact that they offered no flexibility/maneuverability for the actors, I thought the suits in first class were pretty cool.
The dark blue complimented the bright yellow. They consistently resembled the other as uniforms, yet still had small but unique details between them. And they actually seemed like something practical to wear.
Of the new trilogy films, the only suits I can say that didn't look great were the ones at the end of Apocalypse. Though even then, I can't bring myself to say that those were bad.