I was wondering when they'd get to this one.
SCP-2000 is arguably responsible more than any other for the way that the Foundation shifted from single, relatively mundane anomalous objects (like the infinite cake), to wider arcs. Series III, which 2000 kicked off, is where, IMO, the idea of the SCP Foundation as a unified and interesting body of science fiction / new weird literature begins to take shape.
I like pretty much all SCPs of course, but 2000 is definitely an important one.
Surprised at no mention of the original Document Recovered From The Mariana Trench, as despite being by a different author, it's what inspired the creation of SCP-2000.