The full name of this site is “TeX – LaTeX Stack Exchange”. This is fairly consistently used in the site’s documentation whenever the name is spelled out in full — e.g. the tour, the help pages, and more.
“Stack Exchange” or “The Stack Exchange Network” refers to the whole network of Q&A sites on many topics; the company that runs these is called “Stack Exchange, Inc.” These again can be seen both in community usage and in the site’s documentation, especially the legal documentation.
You could thank any of these entities, but I guess TeX – LaTeX Stack Exchange is the one you want.
On a different point: I’m not sure a bibliography citation is the appropriate thing to use, though it’s certainly arguable and will depend on the general style conventions you’re following. Usually citations are reserved for specific documents or talks — “This proof is due to John Smith [Smith 2025, personal communication]” — while for nonspecific acknowledgements, an in-text name without a citation is used — “My understanding of frobnicating gizmos benefited greatly from discussions with John Smith and the Bilbao gizmology seminar group.” Following that principle, I’d usually use a citation to reference a specific Stack Exchange question/answer (as discussed here), but for general thanks to the site I’d just give an acknowledgement in text, and possibly a url in a footnote.
