There are a metric ton of aliens in the X-Men comics. While any moment with the Brood could potentially make the list - and even tie-in with Captain Marvel - none were more prolific as the Shi'ar Empire.
The biggest problem is that the Shi'ar were arguably the reason that the whole Phoenix Saga started, given that it was during the X-Men's space romp with the Shi'ar that the cosmic entity would take the form of Jean Grey and do that voodoo it did so well.
The Shi'ar were also responsible for orphaning Scott and Alex Summers, leading their father to becoming a space pirate, their brother to become a megalomaniac, giving Charles Xavier the ability to walk again, having a massive war with the Kree, and even put Reed Richards on trial for reviving Galactus.
There's so much that they've done that omitting them from every incarnation means that the closest live-action adaptation would be the Centauri from Babylon 5.
Learning to read from his parents' old comic book collection, ink has flowed through his blood since he was born! And now, the self-proclaimed Comic Archivist (at least on Youtube) brings his talents for comic book history and obscurity to WhatCulture! Be warned: he's a Cyclops fanboy and a Nightwing groupie!