Looking back in my sandbox, I'm a little surprised to see the first thing I ever drafted was SCP-2559, albiet in very scant and cringey form.
You'll notice it's not exactly done - I really didn't do anything but build on this for ~80 drafts until I felt it was ready to post almost a year later. I'm very boring.
Looking at it, I think I'd filled in all the [STAND IN TO BE FILLED OUT] sections by the time I posted, so this may be a day early. It's pretty much what the critters would have seen. I've had to take out collapsible chunks so I could stick the whole thing in a collapsible.
If I go way back, though, my first thing I could vaguely call an SCP was a birthday present I made for a friend four or five years ago. It was a physical folder, a mock-up document describing an OC we'd developed together, containing a standard article, a handwritten partially-burned journal and a CD containing interview logs. The actual OC wasn't too interesting (it was basically Dr Jekyll and Mr Hyde), but I'm still proud of what I did there. It's something I want to try again with an existing article if I ever find one suitable.