WHAT MAKES DEOXYS SO COOL, THOUGH?
I'm glad you asked!! I like everything about it, from its design to concept to execution, and all of the actual in-game stories surrounding it only add to how mysterious and awesome it is as a Pokemon. I first latched onto Deoxys when I was around 12 years old and was playing Alpha Sapphire for the first time (my very first Pokemon game!) and got to the Delta Episode (keen readers may notice the connection. Yes. I did in fact name myself after the Delta Episode of ORAS because I love Deoxys so much).
Deoxys was just such a bizarre aberration that hadn't really been seen before- sure, there was the odd mention of Clefairy being from the moon, and there's the Behold Thine Getty Images bizarro God stuff with Arceus that places it on an otherworldly level, but Deoxys was a very robust attempt at making a capital-a ALIEN, something that you could look at and you'd just FEEL the words ALIEN INVADER. I was really into alien and space aesthetics when I was younger (and still am to a slightly lesser extent now), so Deoxys really stuck out to me.
VULNERABILITY ZONE
My obsession with the idea of aliens and fucking off into space is in no small part likely due to my autism and how I related to others when I was younger. I never believed I was an alien, per se (unlike some autistic children, I'm aware that's common) but the concept of space aliens always fascinated me. There was always this acceptance that they would be weird, difficult to understand, and operating on their own rules, and regardless of whether or not they were malicious, they were always the "other". Growing up, it was hard to not always feel like the "other"-- I struggled with making connections and understanding social cues, and didn't have any really close friends growing up. I did, in a way, feel like an alien to everyone I knew, and I'm lucky enough that I have friends in my life that I no longer feel that way with. But because of how I grew up, I think I really attached myself to that incomprehensible "other", characters and creatures that were considered as not quite part of the same group as everyone else, even if they were technically considered part of it. Not merely satisifed with being considered a unique Pokemon, Deoxys kind of calls into question what COULD be considered a Pokemon, and its strangeness helps us define what Pokemon even are.
SOME THOUGHTS
The idea of space invaders and aliens and such might sometimes bring to mind cute and weird little green dudes, or terrifying rows of teeth on a Xenomorph, but aside from this "otherness", Deoxys is also, to me, based on one other thing: THE COOL FACTOR.
SERIOUSLY, JUST LOOK AT IT. Even one of the LINE sticker concepts of Defense Forme Deoxys calls it a "PRETTY COOL GUY". Most of its Formes are made of mostly sharp edges and nasty looks, and while there are a couple of depictions that make it look a little goofy (and I love that), Deoxys just looks fucking SICK as a design. It's able to be humanoid while also having enough design elements to convey that it is DISTINCTLY nonhuman. Having it be designed off of DNA strands also grounds it enough in human biology to kind of create this interesting dissonance- we recognize it as being DNA, but its not from this world, so does that mean that other alien creatures would be recognizable like this?
What does it say about that world?