Why are UFO/Alien easter eggs always the absolute best kind of easter eggs? They're always the most fun to hunt for, especially in games with big maps where there's a lot of places they could be hidden in. Stuff like hidden UFOs, off-limit Area 51/Site 4-esque locations and references, secret government equipment and weapons that you can use, actual aliens, etc. that you can find in the most remote places of the map/level are always intriguing. Even if they're just there for shits and giggles, the feeling of mystery and curiosity that comes with extraterrestrial stuff that's clearly out of place, especially in games that never even hint at their existence, makes it all the more enjoyable. It gets even better when they become an active part of the game once found.
Unlike other kinds of easter eggs, the sole fact that they're often hidden in such a secretive manner that you're really not supposed to find them so easily is what puts them above all the others. Sure some might like the usual big foot, ghosts, hidden boss, pop culture reference easter egg, but nothing beats a good old well-hidden flying saucer crash site.